The risk could be that a competitor could step into the market and use "legacy support" as a positive of their platform. Youtube is weird - especially at that time - since it didn't really have any movers and shakers in terms of either consumers or producers, but if they were selling software that corporations legitimately use (for instance - vimeo offers good streaming video embeds that do "things" to try and restrict the video to a limited audience) then a company using this service to host their streaming videos may be motivated to shift off of that service to a competitor that won't artificially limit the users that can actually use this third party plugin.

Again, it's a bit hard to say here with YouTube in particular, because it isn't a place where work happened at that point[1] so none of the consumers wielded significant power - honestly YouTube may have been fine just say "Welp, 18% of our user base is forcing us into supporting janky old browsers - guess we're losing 18% of our user base and saving a bunch on code maintenance."

[1] Now there are streamers that may legitimately threaten the company if they threaten to jump ship, it's economically weird but beside the point... just, back then there was nothing.