OK, so would people be up in arms had it been LNR who offer no catering and a seat that definitely isn't to 1st standards?
No. There are two separate issues here which can't be conflated. The first is that re-routing should be on the earliest available train, if the passenger so desires.
Then on that service, travel should be under comparable transport conditions insofar as that is possible. If none of the available transport conditions on that service (e.g. classes of travel) are as good as the original service, it is up to the passenger which option they wish to go for.
This is a rare case where a comparisons with the airlines might assist. If you bought a First Class ticket, but your flight was cancelled, you should be rebooked on the next flight. If that happens to be on an airline where there's only Business Class and Economy, it is your choice whether you take Business Class or ask to be rebooked onto the next flight with First Class.
But if the next flight does have First Class, it wouldn't be acceptable for the airline to say "well we'll just rebook you in Business Class, it's close enough isn't it".
Same principle here. First Class does exist on the Avanti service. So any rerouted passengers are entitled to travel in First, not Standard Premium.