We should support altcoins simply because Bitcoin can't be used for everything. Does Bitcoin support smart contracts and tokens like Ethereum? Does it work as fast as Litecoin? Does it have completely anonymous addresses like Zcash? Bitcoin is far bigger than all this coins. They are all used for different purposes by different people. It is really hard to introduce an upgrade to Bitcoin because our community is big and has many different opinion on for example the block size or whether or not we should have introduced SegWit.