The Data-Terminals and mainframe architecture of the time was a technical solution emerging from limitations of the time, bulky and pricey electronics. Also, computers at the time were used for special tasks.
Today, we can fit an ultra powerful computer in your pocket, no problem. You practically can't survive without a smartphone (or computer), or yes you can survive, but you can't be part of society without one.
Today's trend to takeaway the computing power off the hands of normal citizens, when it's so cheap and practical to have it, is not driving by technical limitations (or improvement of technology). It's driven my the goal to extract every bit of money, ownership and finally freedom that we have have citizens.
Why would they give you the option to buy a top of the line 2000€ laptop and let you use it with Linux for everything you need until the heat-death of the universe when they can can extort you of 200€/month instead? Much more profitable. They can change the price whenever they want and you will have no choice but pay, because you can't even find a single DDR3 RAM stick to buy anywhere! Why would they just let you use the heated sits in your car when they can extort you of a 60€/month subscription for it, on top of the 70k€ buying price ?
It's not a technical trend, it's an ideological one.