The difficulty with that idea is that a USAF divested of its older platforms is a dramatically smaller USAF, because few airframes were inducted in the post-Cold War era and both fifth-generation projects were delayed, over budget, and experienced significant ongoing challenges in terms of operations and maintenance costs and future development paths.
One can appreciate where the basic idea came from. The objective was to bridge between the dominant force structure of the then-present, to the equally dominant VLO force structure of the future. That transition was threatened by the lower budgets of the post-Cold War era but, conversely, the "unipolar moment" allowed USAF to divert resources from near-term procurement to longer-term R&D. The difficulty is that those longer term bets have not paid off to the extent that was envisioned.
2021 paper from the Mitchell Institute goes into the arguments against the acquisition of F-15EX, and those arguments are compelling enough on their own terms. Zooming out a little, though, it's more than a little odd that USAF advocates are at all troubled by services (PLAAF/PLANAF) that are implementing force structure strategies that are explicitly rejected for USAF, i.e. the ongoing production of advanced fourth-generation fighter jets.
One conclusion that may be drawn is that these dichotomies that are presented (investing in advanced fourth-generation aircraft vs. investing in fifth-generation aircraft vs. accelerating R&D for sixth-generation aircraft) only present as such because there is insufficient budgetary, institutional, even political space to pursue those paths simultaneously. Using the American nomenclature for the purposes of comparison, China currently has two advanced fourth-generation combat aircraft (counting J-15/16 as essentially one design and production lineage), two fifth-generation combat aircraft, and at least two developmental sixth-generation combat aircraft (that are clearly complementary, rather than one being an alternative to the other). Orders, budgets and personnel can be shifted around between these projects as required. The American system today simply has fewer moving pieces, and so the choices present with much harder-edged trade-offs between competing priorities.