"You want a blue car? Nobody likes blue. Also, if you want a blue car, you can buy this red car. How is a blue car better then a red car?"
Or put differently: Saying one doesn't need one thing, because there is another thing that is kinda-sorta the same except it is different ist a strange argument.
You may like rifftrax more than mst3k, but that doesn't mean that everybody has to share your opinion. Stay with riftrax and let everyone who want's to support mst3k.
You're not losing anything by both existing in parallel.
Disclaimer: I don't know rifftrax and I don't care about mst3k.