The answer to that is well...pretty much everything
. The sub- cruise missile combo is extremely limited in what it can do especially given that it does not eliminate the Find and Fix problem, and on top of that it still has a very long time of weapons on target where you can (and will) loose SA quality in a dense and contested environment especially if the enemy knows what to do and is a "near peer".. The only mission where you may be able to cost-effectively substitute a submarine for a bomber is for strategic nuclear strike but even then most large powers want to (and do end up) investing in a credible triad.
Ever since Vietnam the role of the Continental or Intercontinental bomber at least, in the US has been that of an extremely flexible asset that routinely PIVOTS between being a strategic asset and a tactical assets and does so very well and at a theater combat level does strike at a lower cost per target than any other tactical asset..A submarine with a long range cruise missile can only attack targets that are moderately well defended, are fixed, have an excellent SA available for strike (largely fixed) and are not contested. If you have to have a weapon fight its way to get to a target you have to really design one for it. Hypersonic? Sure..that will add a layer of flexibility, but good luck developing a mach 6 hypersonic weapon that can pick out moving targets...Not going to happen and even if it could you couldn't afford large number of those weapons because of cost per target and the overall cost equation whereas a $250,000 Jammer (GPS) on the ground can create a ton of A2AD and may require you take it out under heavy defensive cover...If you are going to trade multi-million dollar missiles for those targets the enemy wins by just out producing you (classic war of attrition)..
All one needs to do is look at the target sets for the Gulf War, and the Balkan conflict and then find an appropriate multiplication factor because a near peer will field all sorts of twists, and decoys for you to discriminate and overcome, and then factor in that against a near peer your weapon probability of success will become significantly lower due to the simple fact that you will be contested in the RF spectrum, will be contested in the cyber domain, will be contested in space and will be contested when it comes to getting good PNT ability..all of this lowers the ability just as a standard Chinese anti-ship missile launched at a average pacific naval ship is a lot different form a Probability of success pov. than it being launched at a US DDG with all the soft kills and EW tools available to it..In that context calculate the cost of shooting everything down from 1000's of miles away using a sub-launched weapon and factor in the cost of a command level conflict..China and Russia have seen what the USAF could do in the early 1990's and beyond using stand off weapons and the entire A2AD strategy has been designed to prohibit them or anyone else form doing so. IAD's are better protected with much cheaper weapons, a large component of this is MOBILE, even C2C is distributed and de-centralized to a point and there has been a lot of investment in decoys (ONI reports from a few years point to that)..Therefore developing a strategy similar to what the US practiced or practiced in the 90's and 2000's is a poor allocation of resources for the threat has had decades to counter that and has effectively done so...
How will you attack a deployed SRBM, IRBM setup inside the border well protected by a dense IAD when there are decoys mixed in and the entire package is mobile? And decoys are just not decoy missiles, but also decoy radars, decoy SAM batteries and all the good stuff..
..If I were to guess I would guess that the LRS-B (or rather LRS-S) will be more tactical than strategic and compared to the B-2, would carry around 70% of the payload with about 80% of the range ( And the good old Ben Rich will be cursing through his grave saying this is what I designed
the last time around)..All in all a smaller aircraft with a larger payload for sensors than munitions and of course the system to help it both find and discriminate targets and suppress air defenses (RAQ) so that it can operate in a contested environment for prolonged times..The A2AD strategy and the deference value is different from those days of the cold war when the primary threat and mission was to support the SAC in its strategic mission. Today the B-1 does CAS..and the problem is not of launching a nuclear salvo against a threat (that is treaty limited anyway) but to fight your way into a contested environment, once there figure out and discriminate targets, pick them, hang around and do strike..from an A2AD environment quite a bit different and significantly more complicated to the B-2 problem which had a primary mission of going in and going out..while the LRS-BOMBER may do that the support vehicles will most likely have to stay in the contested environment for longer and hence unmanned and optionally manned assets.
Therefore based on all this a blanket statement that the IAF should get bombers is of limited value. First and foremost one needs to come up with what those bombers will be doing. The most logical answer for me is MARITIME STRIKE using stand-off weapons. That makes sense but if there is a mission to penetrate deep into the eastern border to inhibit the offensive capability of China during war, then that will be a much larger task and will require a lot of support capability before the strike bomber is even conceived. China is developing something to keep the USAF, USN and Japanese forces out of its A2AD network (of course those forces aren't going to invade china but here it is keep them out of its sphere of influence and the way China is doing this is by having a large maneuverable A2AD that it can quickly setup and impose a very very heavy cost to any force that wants to operate within that area of concern) so outside of deploying nukes the IAF would have to come up with novel strategies to develop some sort of capability to take the fight and begin to destroy some of the offensive capability...T