I would hesitate to look at this through the perspective of the Redcap himself since he's going to shuttle as many people as he can to make the most possible money. What's good for him isn't necessarily what's good for pax with special needs.
But running with your example...
You as an able-body are taken down to the train either first or with the first group. Sitting upstairs in Boston CA is a frail elderly woman with many bags, waiting for the Redcap to return. She may or may not even be part of the second group. Let's say that I and another able-body were queued next, meaning she would be brought down in the third group. By this time, it's possible and even likely that general boarding has started from the CA. So, by the point this woman is boarded with the Redcap, the cabin is already bustling with people and bags.
So your argument seems to be that the situation is OK because the Redcap did eventually get her to the train. But this is a person who truly did need preboarding because she has heavy bags, moves slowly, and requires special handling as a frail senior citizen. I personally would feel rotten about myself if I sat in my seat and watched this all go down, knowing that I was boarded 10-15 minutes before her. It's not far fetched -- I saw something similar happen yesterday while waiting to board #8 in Seattle.
If indeed there is a "frail, elderly women with many bags" sitting in the CA who is not taken out with the first group that each redcap takes to the train, then it is because she did NOT ask for help! It's really that simple. Especially in your example of Boston's CA. Boston's CA simply isn't that big or that busy.
That frail women would have to walk right past the Redcaps on her way to the lounge, not to mention climbing the steps with all of her bags. If she does get to the elevator, well then she needs an Amtrak person to key the elevator to get to the lounge. Nine out of ten times, that "Amtrak" person will be a Redcap. Then upon checkin, the attendant would ask her if she wants a Redcap. At 51, I'm still quite able bodied, and even I get asked every time I check in if I would like a Redcap. So again, if she's left sitting in the lounge, then there are only two possibilities. Either she didn't ask for help or the Redcap forgot her. And they're pretty good about not leaving people behind, especially in Boston where there simply isn't a large crowd.
Additionally, on more than one occasion I've been taken to my train in the company of many others. Sometimes as many as 7 or 8 people, not all of whom where even in the lounge. Each Redcap doesn't take just one person at a time, unless they only have 1 person who requested help. And there are typically at least 2 or 3 Redcaps on duty, each with their own group of passengers.
Once at Penn, I requested a Redcap which I don't normally do for myself. But my mom was with me and we decided with our luggage that it would be easier. Mom's 78 now, but at the time I think she was 75. Our Redcap took 4 other people with us, one of whom was in a wheel chair. I actually carried/rolled 2 of our bags, because his hand truck was full. He pushed the wheel chair with one hand and pulled his hand truck with the other. A second Redcap also worked that same train.
In NY & DC, again most agents will ask if they think that the person might require assistance and anyone can simply ask if they want one, without regard to ability.
In Philly, again the only way to the lounge is to climb stairs or get a Redcap to key the elevator for you.
In Chicago, prior to the general boarding announcement for any train, they make a pre-boarding announcement for anyone wishing Redcap service to come to the front desk about 5 minutes before they make the general announcement and take people out the back door.
As to your Seattle example, without knowing the details and when that person arrived and asked for help, there is little to debate. Could it happen on a rare occasion? I suppose that with the right combination of mistakes or a Redcap being out sick, sure. But in general, no one is getting left behind who wanted help because some able bodied person asked for a Redcap.