I don't see the problem with rent-a-players, especially in an environment where transfer prices are pretty stable.
Assuming the owner doesn't qualify as a day trader, transfer tax on a 3-week hold is what, 85%? So if you buy and resell the player for $5M, and he has a weekly salary of $200k, then you just paid $2.1M for 3 weeks of your mercenary. By contrast, your competition has the option to buy-and-hold similar players with a more well rounded skill profile who only have a weekly salary of $150k but sell for $7M because they're more balanced (or whatever situation actually prevails). It's another trade-off. Do you want to pay $700k a week for a 3-week rental of a huge salary player? Or do you want to build a team you can buy & hold, and have a better long term result?
To me, allowing people to have meteoric flare-seasons where they rent the best roster money can buy and then wind up pissing away all their capital is a necessary and beneficial tactical option for this game. Otherwise, there will be dramatically less turnover among the top teams, and a new owner is going to be a lot less likely to get drawn into the game to the point where they'll stay, because the league structure is so fossilized that the new owner doesn't see much of a chance to ever make it into the top two league levels.