You've only looked at the cons.
With argument b, you cannot speak for everyone. I don't think very many people would actually care, or would look at it with the same perspective.
Some will say "it was exclusive for long enough". Some will say "it's a better decision for the player base overall, and therefore the game".
It's also not a net loss if one or a few people lose this ephemeral trust in Frontier. If the result is that the decision impacts their active player numbers or sales positively, or at least not negatively, then it's not a loss for their business. How and why people go around deciding whether or not to trust a games development company is beyond me though.
As far as the pros go, people who play Elite love spaceships, and consider no amount of spaceships to be enough spaceships. Giving them access to the MKIV would give them something else to try out in the game, which is a positive even if they don't end up liking it or finding a use for it.
On the flip side, how many people will lose trust and quit playing if they don’t get access?
And if trying out a new ship is the only thing keeping them playing why should FD care, because they will try it and get bored and quit anyway