Well, I think we have multiple models because of regional radio frequencies that regions use. It's not like manufacturers want to make over 9000 models, they will lose money. For example, over here in the USA, we use 1900MHz and AWS (1700/2100MHz) radio frequencies for 3G, and while over at UK might use something like 2100MHz, and Asia will use whatever frequency.
That sounds like my dad too. He used to have a old nokia (not the Nokia 3310, as it won't work over in the USA), and some other old phones. How he has a smartphone, and he owns an iPad.... But he uses it just for news, weathers, movies, that's all. Now my internet connection is being slowed down, originally I am the only internet user, then my dad and my cousin sharing my connection. Man, I wish I can get a faster connection in the US like in the UK. Only got a 15mbps connection. It's just how shame that US is one of the most advanced countries, but in terms of internet speeds, it's ranked so low in the developed countries, and we created the internet.
Of course, that will be a selective amount of people (like you and me), will still prefer original desktops and laptops. It just feels weird typing on a touch screen, but although you can get optional keyboards and mouse for tablets. But not alot of people are power users, that just want something that will just work for them. As desktops and laptops' sales decrease, it's not up to the power-user minorities to decide for the manufacturers to continue making desktops or laptops, as they will not profit the manufacturers in the future (hopefully not). Maybe we will have to buy server parts in the future since manufacturers won't provide desktop parts lol.
I don't really feel like SSD prices have been dropping alot in the recent years. I got my Corsair Force GS 240GB 2 years ago, and I got it for $200 US dollars. Today, probably around $160-180. As SSD prices drop, HDD becomes less significant, although they are good for storage (for now). Ultrabooks... I feel like they are being left out too... Many people would still get a tablet.
Future of desktops and laptops? We don't know. All we can do is just speculate. For sure I will continue to support desktops and laptops for the next 10 years, as I will build a gaming desktop maybe sometime in college, and build a NAS with my current desktop which my future gaming desktop or laptop will replace. I can't imagine what our next generation of people will use as mobile computing is going so advanced these days.