and is probably the future "main" Minecraft edition anywayProbably never
The Java version has some very things that the C++ version does not have: a large modding community. This alone will prevent the C++ version from becoming the 'main' Minecraft version. Until it has a good, functioning modding API with a lot of mods ported, people MIGHT go over the the Windows 10 edition. But all the Linux and Mac players can't, so even then the Java version will be used a lot.
Never? Not quickly and not too soon for sure, but "never" is a bit of a strong term, and I can't say I personally agree with it.
PE is confirmed to be getting an official mod API, and unlike the Java edition's fabled mod API, I think this one will come much quicker, given the recent speed of MCPE updates and the much larger dev team behind it, not to mention the PE codebase being more clean & optimized than Java MC, which would probably help a lot. I think the modding community would be VERY eager to develop for a MC edition that runs on pretty much everything except the aforementioned Linux & Mac, and has both cleaner/faster code AND an OFFICIAL mod API.
And as for Linux & Mac, PE will almost definitely get ported to both at some point, and as for Linux, it's already technically possible thanks to this: https://github.com/MCMrARM/mcpelauncher-linux
So it wouldn't be that hard of a port. I think the only reason it hasn't happened yet is that it's not a top priority right now (feature parity is), and also that PE needs an official account system. (And judging by the Realms beta test and Win10 Edition, it looks like Xbox Live may be that system.) The devs have already stated they want Minecraft to run on everything, and so while it may take a while I highly doubt that PE will never be ported to those 2 OSes.
So will MCPE become the main edition? Absolutely. Definitely not now (due to the reasons you've stated), but both obstacles are almost definitely going to be overcome at some point or another. But I guess to know for sure, we can only wait, hope, and see...