Whilst Google may well be in a better situation, I'm not sure I agree with your reasoning. Google are pushing people into their cloud products far more so than MS. And MS's products, including Office, are cross-platform too (I have One Note and Sky Drive for my Android phone, no different to WP; they've released for Macs for years).
"Home computing is with tablets and smartphones because they are simple to use."
Some people may prefer tablets, but as much as I love my Nexus 7, I still prefer using a laptop. Simpler to use with its keyboard, touchpad and larger screen. And I like just laying it on my lap, rather than have to hold it. And typing on a touchscreen is unbearable. Whilst a few people may use phones at all, I don't find it better. I think the biggest advantage tablets will have for their prevalence is having a far lower cost than laptops. But despite being around for years, we've yet to see the death of laptops - if everyone preferred them, why aren't they already using them, in place of laptops rather than in addition?
For all the people I know buying phones and tablets, they seem to be getting these gadgets as additional devices.
"I'm not saying PCs will disappear, nor am I saying Microsoft will, just that until today, people only had PCs to surf the web, go on YouTube and write their emails/tweets/sms."
People used PCs to write SMSs?
Also remember though, up until the mid-2000s, people weren't doing things like Facebook, Youtube etc at all. There was actually only a relatively small window of time where both people were buying technology to do these things, and when laptops and desktops were the only choice.
So yes, we may find the market size drops a bit - perhaps as much as say 50% of the set of people who only use a computer to browse Facebook/Youtube no longer need a PC. But you'll still have the people who prefer laptops; and you'll also have all the reasons that existed before the mid-2000s for why people had PCs - such as for word processing, printing, games (not all of which translate well to tablets), working from home. I find it odd that on forums where people seem to hate the idea that Windows 8 "only works with tablets" (even though it isn't true), also seems to have the prevalent view that laptops are dead, and we should be doing everything with small phones and tablets.
Also the laptops vs tablet argument is starting to become irrelevant, with tablets that are convertibles and full blown PCs. Did we have these debates about the death of the desktop, because laptops were appearing?
Long term I think Android will be far more prevalent than anything else, because it will be more suited for an increasing number of devices that become computerised (TVs, cars, home appliances, etc). But even though my British Gas boiler now connects to my wifi network, that doesn't mean I'm going to throw my laptop away to do computing on the boiler or fridge-freezer.
I'd also add that Apple would have more to lose if people switch from laptops to tablets, as the strongest market for Macs is in the ultra-portable segment, competing directly with tablets.