F-35's problem is that it replies on future technology that isn't achievable. It is kind of "we will have it when we need it". This is an over-confidence without grasping of reality. It is the same as wonder weapon mentality.
The current F-135 engine provide maximum 191KN thrust, the ACE is 200KN, the improvement of power isn't much, but only the fuel efficiency. The problem of F-35 is however more about total power rather than efficiency. One example is that the cooling power of F-135 was designed at 15kW, but F-35 today uses 30kW already, block 4 need 40kW. No matter how efficient ACE can be, the total power is far less than needed.
F-119 is 135kN, two would give 270kN far more than 200kN of ACE. Essentially the problem is F-35 being a single engine airframe trying to do a twin engine airframe's work with unavailable engine technology breakthrough.
Therefor, F-35 is a dead end. It is the same mistake as railgun of Zumwalt. Also similar to EMALS. All of them are designed on the hope of "the critical technology challenge will be solved in time, one way or another".
B21 isn't a fighter, so it's number of adoptation isn't any indication of less F35. However, NGAD is. Let's wait and see if US makes the same kind of higher-than-realistic expectation like they made on F-35/Zumwalt/EMALS.