I dunno. I don't have any sort of inside info (of course), but it just seems weird to me that the US designed aircraft in the late 1950s (XB-70 Valkyrie) that could cruise at Mach 3+, but now our stealth capabilities are so good (or drones so effective in replacing reconnaissance aircraft) that those high-speed capabilities are no longer required.

I can only offer speculation. The subject of mysterious military aircraft is of course a rumor mill and field of conspiracy theories like no other. :-)

https://nationalinterest.org/blog/the-buzz/rumors-secret-war...