Ahh, that's a very big and important question. It all goes back to Beardy Branson wanting to run three-class tilting trains with exclusive rights to run on the WCML. You might recall that the late-unlamented Railtrack signed on the dotted line then discovered it couldn't deliver, and went bankrupt.
The West Coast Route Modernisation that took place under NR ownership had the shadow of the Virgin/Railtrack deal hanging over it at all times, so - with DfT holding the ring - NR produced an upgrade which still massively favoured Virgin by effectively barring any other operators from running at 125 mph on the WCML. In return for this, Virgin dropped its legal case againts NR as successor to Railtrack.
With the passage of time, the miracle of tilting trains can be viewed a bit more critically, and the possibility of running non-tilting trains advanced more quickly.
So you are saying that Virgin insisted on tilting comparable infrastructure to lock out competition, and that non tilting compatible infrastructure would have achieved the same result? If so, how? Even with the latest proposals, it seems we won’t get as good as what we enjoy now in terms of speed and comfort.
Also, the tilting infra doesn’t seem to have stopped GC from setting up an open access without tilting (albeit, covid has strangled that at birth). And what’s to stop an open access operator acquiring tilting stock?
Finally, whatever the history, we are where we are, so why are we now reinventing the wheel? The tilt compatibility was completed at huge cost. Non tilt can’t match the speed and comfort. Avanti should have (or should have been compelled to) order tilting trains.