Their iMessage is just like a Facebook Messenger or Hangouts... the part that they did right (and wrong, because a lot of people can't tell the two appart) is seamlessly merging that with SMS/MMS. Problem is you still need a valid phone with a valid SIM for iMessage to work with SMS. Allo won't be any different. Only VoIP services, like Google Voice or Skype Out, let you text from any device without the need for a phone with a SIM card.
Also, you do realize that you ARE sharing your phonebook with Apple, right? It's not as 'demonized' as Facebook, but they're no saints. How else do you think they 'know' which of your contacts use iMessage and which are text-only?
Not sure if Allo would do that, but Google Voice already does... and in non-manufacturer-specific hardware, unlike Apple's. You can do it via an app or browser anywhere. iMessage does the SMS/MMS sending/receiving on their end and then sends it back to your iMessage client via IM. That's why it works, and that's similar to how Google Voice works (but in the case of Google Voice, you get the number directly from Google, not the carrier).
As for a Windows 10 app, I wouldn't be sure because of how apathetic devs are to that platform lately, but at least you will be able to use it in a browser.
You can do it now even if you don't yet have the play store with the pushbullet extension, that's what's I've been using works great and it links to your phone so every text you get shows up on your computer and you can also initiate a text from your chromebook.Will I be able to use a Chromebook for text messaging when Android apps will be available for Chromebooks Thanks.