I'm confused because for as long(2014) as I have been in cryptocurrency space there has always been an understanding that in case of a fork, the longest chain retains the name. Why does that have to change now?
The difference is that the type of "fork" you're referring to probably meant a temporary split within the same consensus rules, where one branch would eventually be orphaned off. This situation is different because the "fork" breaks the consensus rules -- it removes the block size/weight limit and replaces it with a higher one.
In the first scenario, we would expect that once one branch was longer, that miners would converge on that branch (because all nodes on the network would follow the chain with the most cumulative work). In the second scenario, the fork is incompatible with most network nodes (Core), so it will permanently split from them. It is a totally different network with a new 21 million coin supply. In this case, both chains could survive, and we could have two "Bitcoins" (or rather, one Bitcoin and one shitcoin), each with a 21 million coin supply.