Very true. The other day I read or saw somewhere how the UASF already started asking question to industry on 'what next' even before one of the teens was yet to even fly for the first time and other was only undergoing flight testing. Once I collected all the airplane documentaries that I could find and went through them. Recurring theme was how the next plane was always next in line when the current one was barely inducted. The cycle time has elongated over the decades for various reasons but the cycle goes on still. This is true to large extent for other nations such as Russia or France or UK as well. But due to ease of finding information and sheer vastness of efforts, its easiest to notice the continuation in US Aviation history.
Jay, if you go through the book on the F-22 written by its program manager you'll see that the long lead technology investments in support of the ATF actually date back to late 1970s (I have a post quoting directly form the book here somewhere) and by the mid 1980s this was a formal program with fairly large contract awards. Same now. Last month , GE was expected to complete their design review of the XA100 (definitive designation if selected will be A-100) and expect to have the first of the three prototypes begin ground testing by the end of 2019. P&W have been less forthcoming on their prototype but they wouldn't be that far behind with their progress on their engine either given that their contract was for similar timelines. This at a time when the F119 and F135 are amongst the most advanced military engines in the world.
By 2021 they should begin flight testing of one or both engines if they want to proceed with that (as opposed to further EMD awards). Propulsion has traditionally taken the longest time especially if you are looking at a leap ahead capability and not re-purposing existing propulsion solutions. Same with other areas of investments that are at low technology readiness such as directed energy weapons which will also be demonstrated on an F-15E in a couple of years time. The fact that you invest in these capabilities isn't proof that you are not satisfied with the capability you are fielding at the moment but rather indicative that you are actively pursuing a strategy to have things ready for the next replacement cycle which for the USN is to replace its substantial Super Hornet and Growler fleet and for the USAF to field a new Penetrating Counter Air aircraft to complement its long range strike and ISR fleet.
The Soviets did it the same way as they were locked in a cold war with the US and NATO and the Europeans too until the peace dividend and the downsizing which meant that they did not take the higher risk and fielding a Low Observable, 5th generation program.