If this were FUD, the network wouldn't be having problems.  The solution to this problem isn't writing software, the problem is not fundamentally a software problem.  It is a distributed systems problem and a human problem.  It requires leadership. The code is the easy part.

If there were a clear protocol specification there would be a simple way to declare the other wallet developers out of bounds.  Even without a protocol specification, bitcoin core could be changed so that bitcoin core nodes would not relay ill formed transactions, nor accept and propagate blocks that contain ill formed transactions.

There has been ample time to phase in a solution to the problem.  Some old clients would cease to work.  That would be good, not bad.  People with funds in an obsolete wallet could always export their private keys and sweep them into proper new software.  (This part of the argument is the "leadership" problem.)

One consequence of some of these software implementations being obsoleted would be a demand for a firm protocol specification that could be discussed and debated independently from any code base. With a specification there could be the beginning of work on protocol verification and proofs of correctness.  Accomplishing this would require creating some kind of governance structure, such as is used to manage the IETF.  Adults would step into positions of leadership to replace children...