I think these are all valid points, but I'll take the other side.
The event is for players that are currently amateurs. I don't think there should be a second test of "purity of spirit" with regards to amateur golf for player or captain. If I'm not mistaken, several players will be professionals next week. Should they not be selected because of this? I don't think it should be different for the captain.
In this sport, there are no top level amateurs, per se. There are just those that aren't professional yet. It's pretty universally accepted that the best way to get really, really good at this game is to play in competitions that require amateur status; that's the training ground. So, players maintain amateur status so that they are eligible for those events, and when they feel they're ready, they go pro. If they could get paid and still compete in those events, they wood. If they don't think they have the game to make it as a pro, they keep playing as an amateur. But that's a tough road, because there's always a new batch of (future) professionals against which to compete.
Historically, the amateur game was for those that could afford to play without pay; the professional game for those that couldn't. To me, this is not golf's finest historical precedent. The present system gets away from that, because age and ability, not wealth, are the dividers between amateurs and pros. Retaining a bias against players that had at one time been professionals would support this idea that those that can play as long time amateurs are somehow "more pure," which is an idea with which I don't agree and which I think undermines the progress of the game.
Finally, there is the practical problem of a lack of available candidates if the USGA wanted life long amateurs. And, even a player such as Bobby Jones finally went pro, does that mean he sold out? I don't think so.
As I was watching today, I was thinking it'd be fun to watch a Walker Cup for Mid Ams. That would be full of players that were either never pros or had brief and unsuccessful professional careers. I think that event would be fun, and the golf might resemble the game that the rest of us mortals play.