I should point out that no cable company scales their network on the assumption that 3 top tier users will be maxing out at the same time.
Ironically, I remember reading some American cable dimensioning data that said *precisely* that: 3 users on the top tier.
In the example you quoted that would mean VM having the area running at no higher than 25% on the off chance that 3 x 300Mb users all decide to download at the same time to ensure no visible contention.
I'm not sure it said that you have to run with sufficient /unused/ capacity to cater for 3 top-tier users, just in case they start up (which sounds like what you say above). I'm sure it just said that the segment has to have that much capacity available in total, include capacity used by the other 2,000 "normal" users.
Essentially, that dimensioning rule tells you that, if you have a segment capable of 450Mbps aggregate, then the top tier should be no more than 150Mbps. If the segment is capable of 1Gbps aggregate, then the top-tier should be no more than 333Mbps.
Now, to be fair, it was something I read a few years ago now - back when the 150Mbps tier was new. And it certainly wasn't given as a VM rule.
Your alternative sounds like a perfectly feasible alternative, though much harder for us consumers to gauge.
I assume that BT will have to start working to similar planning guidelines when it comes to backhaul in the access network - especially G.Fast nodes (at first) and FTTC nodes, as that sustained load builds some more.