It's not about Marsel, it's about the rule, which should be changed. I don't think a player who retires in qualifying has earned that opportunity the same way a player who finishes a match does, also, that player is injured a couple of days later, they are going to be running around like normal again? So when you look at that aspect too, it's like, if you're unfit for the qualifying, then you're definitely not fit to play in the MD.
I disagree with your logic that a player who retires doesn't "earn" the LL opportunity. A player earns their lucky loser status by winning their previous rounds to get to the final round of qualifying. The do not prove anything extra by staying on court and playing to lose that final round. You may argue with the rule, but even if the rule is changed, players with chance of a LL spot who get injured will just finish their matches. So instead of retiring at 4-0 down in the second set, Marsel would have lost the second set 6-0 in his final round of qualifying.
In the end, if a player is in no shape to play his main draw match, then I think that player should pull out of the tournament himself and will be replaced by the next LL. Marsel was clearly in match playing shape and there is no reason why he shouldn't have gotten the LL. He got himself there by keeping his ranking high enough and getting to the final round of qualifying.