What I’m saying should be in place isn’t “we might take it”; it’s a charge for baggage, to encourage people to pack efficiently and to raise revenue, and a strict limit per passenger. That way there’s certainty over what can be carried and, if you can’t carry as much as you want, you simply don’t take the train.
The problem with this is that it defines the train as a simple adjunct to car ownership, which isn't a policy I could support. With the higher cost of cars and insurance, we need to be pushing towards a combination of public transport and cycling being a
replacement for the car. Thus, it needs to be possible, for instance for students going back to campus a few times a year, to pay to carry more luggage.
I don't see much need for a hard limit as there's a practical limit to what one person can carry, which is probably around 2 big bags and one small one on your back*, maybe plus incidentals like a handbag which don't really make much difference. I did move to Hamburg for a year with two cases which were probably the full NR size - 90x70x30 - and very heavy - and I could barely manage them, and I'm a very big guy. But hardly anyone brings that.
* Which is more than LNER nominally allows, but Avanti appears to allow that.
At the moment “we might take it” is exactly what we have in place for bikes, for example, on services that don’t have cycle reservations; as a cyclist there’s a non negligible chance you’ll be turned away from trains on summer weekends due to demand.
My personal view is that aside from stuff like Merseyrail where you'll be waiting 15 minutes at most and cycles by the doors are tolerated, cycle reservation should, for that exact reason, be mandatory on all trains.
It’s going to become a bigger problem in future as numbers rise - keeping in mind the intercity fleet we have in this country isn’t going to get much bigger over the next 30 years, and is already very crowded. Hence Lumo’s approach is probably trail blazing. There is already scope for baggage restrictions in the NRCoT, so they could easily be made more widespread.
I'm not sure it is, because with people getting bigger I don't think you're physically going to get many more seats in the existing trains - for instance 3+2 on UK intercity trains is just not viable. A luggage rack that will take maybe 8 large bags is only two seats. If anything regional trains will end up with more standee space (so will look more like Japanese trains or London Overground, i.e. fewer seats, probably narrow 2+2 or longitudinal, and lots of standing room), and all that open space makes luggage easier, not harder. If anything the trend seems to be to lower density seated interiors like the Class 195 or 397 but with more standing room.