I wish I shared your optimism.
You only have to look back at older and now rare Rovers to see that values have not exactly skyrocketed - a 50 year old P6 averages about 5 grand in value:
https://www.hagertyinsurance.co.uk/p.../1968-Rover-P6
That's a long time to wait and a lot of money to spend to see it just over double its original purchase price.
... and a 30 odd year old SD1 is worth barely much over £4000:
https://www.hagertyinsurance.co.uk/p...1982-Rover-SD1
Of course, if people want to sink money into keeping old cars running, then that's their choice, but let's not be any under any illusion that there's sure fire money to be made as they become rarer.
Getting back to the topic, I've no doubt that the cars I have sold over the years would now be virtually worthless if still on the road, and there's probably a good reason for that.