If we want to do this kind of lockdown
For the record, that's a mighty big "if".
It wouldn't take much at all to expand that to every machine; all it'd take would be MS adding "in order to keep machines secure, don't allow disabling Secure Boot" to the Windows Hardware Certification requirements
And the resulting monopoly-related lawsuits in every nation that would support them, not to mention almost inevitable regulatory action in jurisdictions like the EU, would most likely be the final nail in the MS coffin.
Even if that didn't do for them, Intel and the major manufacturers of Intel-related motherboards and other hardware within the same architectural family are already under pressure from tablets (most of which are sporting ARM-based hardware) at the casual end of the market. The last thing they want to do is put all their eggs in one basket, particularly a basket as wobbly as Microsoft has been in recent years.
There are so many existential threats to the businesses that would need to participate in such a move, and so many well-funded organisations including many in governments that would have a lot to lose, that I still think it's completely unrealistic for the mainstream Wintel ecosystem to go that way. If anything were to lead to that sort of result it would more likely be a steady creep from the direction of smartphones and tablets where relatively closed and inflexible ecosystems are the norm, but even there the signs are that the initial glow is fading as users both become more aware of the pros and cons of such devices and tire of the cost and hassle caused by the lock-in effects.
The coming war on general-purpose computing and The Coming Civil War over General Purpose Computing are a good idea to read.
They were thought-provoking articles back when they were written, but again I'd say the recent evidence is that people are increasingly tired of these games. A new generation has grown up never not knowing what it's like to have their own PCs and consoles and mobile devices, and fast near-permanent Internet access, and a huge range of software available at the tap of a finger, and all that comes with this kind of technology. They've also grown up more aware of related issues like privacy and security, and wise to a lot of the problems that caught older generations off guard, even as the patience of the older generations themselves is wearing thin and they become less tolerant of the ever-worsening experience as tech businesses try to squeeze ever more profit out of them.
Consequently, there's been a lot of talk recently about things like on-line privacy and ad-blocking. Perhaps more telling than the talk are the moves by some of the biggest businesses in tech to actively support such things, even if means shifting industry norms or taking on governments. In fact, there is even a hint that some in those governments are finally becoming aware of the issues -- there have, at long last, been some substantial steps recently to bring copyright laws and on-line consumer rights at least a little closer to the 21st century in some major jurisdictions, for example.
I do think the writing is on the wall for some tech firms at this point, but from my perspective it is because their customers are becoming less tolerant of junk and starting to demand better quality for their hard-earned cash. Firms that ship software that doesn't work or causes security problems, businesses that leak personal data like a sieve, content distributors that try to double-dip with subscriptions and then ads, communications networks that over-charge and under-provide, on-line businesses that offer minimal customer service... All of these are increasingly on borrowed time unless they change their ways, and that's just in the B2C world. As soon as you go B2B, there are many more examples of long-standing schemes that are under threat in our increasingly open and competitive world, and consequently businesses are likely to be even less tolerant of attempts to lock down what they can do than private individuals.