Having Ryan Reynolds as Deadpool in the MCU likely won't mean the entire Fox's X-Men franchise will be cannon with the MCU...

canon*, and that only makes his movies canon, even though they are apart of the X-Men Movie Universe until further notice. I suspect Deadpool won't join the Marvel Cinematic Universe anyway. We'll see.

Well canon, not exactly, but it will establish it as a part of the Multiverse tho.

More like etablish it as a known earth to MCU residents, since they're technically in the same multiverse already. If it were to crossover, it would be retroactively added into the MCU or in an alternative universe, making the MCU 5 earths altogether (technically we have the 2091, 2012, 2013 and 2014 universes already).

For me, the mere fact of having JK Simmons as JJ Jameson puts Tobey's Spider-Man as a part of the MCU multiverse. 

Well, no. It's just J.K. Simmons playing the role in two separate universes.

Man, I hope against hope they never connect any other Marvel universes to this one.

I know your feelings, but the simple fact of bringing him establishes Raimi's Trilogy (best Spider-Man...) as a parallel Earth to the MCU and I am very aware that is the intention at some point. Multiverses though. And that is okay with me. They will never cross or affect each other. 

I don't really see how it connects them. Stan Lee appears in plenty of other Marvel films as well as this one, but it doesn't connect them.

Who knows 🤘