Also, for those who think SC6G is a replacement for F-15s: the planned replacement for the F-15 is the F-35. The USAF still nominally has a 1900-fighter goal, which nominally is covered by 1763 F-35s and 140-ish F-22s.
And if the SC6G is to follow the F-35 into production (well after 2035) it is ridiculously early in the game to be funding prototypes.
'6G' is intended to replace the '22 in USAF service, and the '18E/F in USN service. YF-22 (and YF-23) first flew in 1990, it took 15 years from that milestone to being officially accepted into service. X-35 (and X-32) first flew in the latter half of 2000, after the contract was signed in 1996. So, I'd think 2015 is
exactly the time the US needs to start funding 6G prototypes - if not a few years too late. Even a 'crash' program is going to see any G6 fighter struggle to enter service in ~2030, even if unencumbered by any notion of USAF/USN commonality.
I wouldn't argue with the fiscally challenging nature of replacing multiple platforms though.