I do not think having an amenity that has long been offered to travelers needs extensive justification. I think, rather, the burden of proof should be on those intent on removing it.
There's a difference between keeping the amenity as-is (particularly when any additional capital cost is a ways out) and building out significant new infrastructure and/or staffing to expand the service, or when large investments to keep the service come up to question its need at that time. I wouldn't advocate for removing it tomorrow, but I'm also not convinced that long-term it should be a place where capital investment is expended, and as new passenger cars are ordered there should be a focus on making sure luggage storage is available in-car (and not require checking in luggage.) At minimum, the costs should be compared to see what (if any) savings would be found by building train cars with full luggage storage on-board (including additional cars to make up for lost seat capacity as needed) versus operating checked baggage service at different levels of stations having baggage service (including what the current cost is and what the cost would be if we allowed trainside checked baggage at all stations without staffed checked baggage service.) Maybe the cost is more negligible than I think, or the cost difference favors checked luggage options.
There's also a lot of Amtrak trains that currently don't have checked baggage service, especially when you narrow that down to just count trains where checked baggage is carried with the passenger on the train. The NEC has very few trains that carry checked baggage, and none currently go north of NYC, and it's not such a critical feature that Amtrak is obviously hurting because of it. Same with a decent chunk of the state-supported trains. Maybe some of it is self-sorting, and some of those passengers do have the option to check it on a separate train, but I would suspect that having to either drop off luggage many hours in advance or have to head back to the destination station many hours (or even a day) later to pick up luggage would rule it out for everyone but the most adamant users of checked luggage.
All that said, a lot of this is just me rambling and speculating on the internet. It's not something that outside of a rail forum I'd advocate for one way or the other, as I'd rather focus my public efforts on items more important to me like increased frequencies, new routes, or even on-board wifi on the western LD trains (yes, to me that's more important than checked baggage, and I've seen more complaints about that on non-train sites that I frequent than I would've expected.)