The MiG didn’t reach 37km in level flight, which is probably where Mach 15 @40km comes from. Instead the MiG was doing a nearly parabolic trajectory starting with Mach ~3 worth of kinetic energy.

The NASA X-43A hit Mach 9.6 as an air breathing engine (test used a rocket at lower speeds for cost reasons) which in theory should be capable of ~65 km assuming it could survive a similar maneuver. Actual limits are heavily influenced by how much thrust you can generate while slowing down etc not just max velocity.

So yea no actually built air breathing aircraft can hit space, but it’s within the realm of possibility.

Nobody has been able to build one, but I'm not aware of any proof that it's impossible. You need some way to build an engine that doesn't appreciably slow the air that's passing through it.