Can you not just clone the shitty product so existing training is still useful? Do software companies enforce copyrights on things like the location of buttons and the names of menu items?
Patents are probably the most powerful tool for entrenched players here. Adobe patented several GUI conventions including their most common shortcut key commands in Photoshop, so that competitors would always have a slight learning curve for new users.
Occasionally more aggressive companies will come after a cloned product with a lawsuit, for instance Facebook sued a number of early competitors for various intellectual property and trademark reasons. Doesn't hold up in court publicly but I imagine there have been a lot of backroom settlements made this way over the years.
A clone of a crappy product is at best a crappy clone.
What makes bad products bad is precisely the stuff you're trying to copy. Any clone just inherits the bad parts then inevitably adds other bad parts behind the scenes or misses random features. (Hello Open Office).
Sometimes that institutional knowledge is knowing how to work around bugs or deal with a crappy UI efficiently. Sometimes that is where a worker's own perception of where their greatest value lies so they'll gladly resist or sabotage anything that may be better (on top of a general resistance to change).
Occasionally more aggressive companies will come after a cloned product with a lawsuit, for instance Facebook sued a number of early competitors for various intellectual property and trademark reasons. Doesn't hold up in court publicly but I imagine there have been a lot of backroom settlements made this way over the years.