1 > Why would we change the definitions of words . . .

In response to organized and institutionally-driven pressures. We often hear the argument language evolves (for instance) in assertions like "TWAW." Language does evolve quickly in areas like slang and tech neologisms, but "woman/man" etc. are core terms describing foundational realities. There's no way the anglophone public would just absorb an accelerated change in the definition of these words, any more than they'd absorb a wildly variable new definition for "food," "river," or "tree" -- the foundational lexicon is too old and well-tested. It's a forced adaptation that negates consensus and subverts reality, and activists who blithely assume that linguistic evolution = whatever changes they want to force into usage are courting huge pushback (and, worryingly, backlash). Pick a modifier or improvised word to describe the new reality for the 1% -- subverting basic language isn't going to fly.

2 > does it not make more sense for them to accept the definitions as they are, but explain how they fit those definitions?

To my mind, yes. Modify or improvise on the old terms -- that's in line with how language evolves. "Trans" itself would be a very strong term to build on or use as a modifier; most people are already familiar with it, and it's a very neutral prefix with no ancient linguistic baggage.