It would just need a major adjustment in the balance sheet to recognise that players could be sold at a much lower fee, they wouldn't be able to hold players at a massive value and every club would be adjusting at the same time so no unfair advantages.
Problems come in if other European leagues don't do the same. That's where American sports have an advantage in a closed system, teams and players have to suck it up and accept the rules.
I get your point in terms of competitiveness as everyone wold be doing it at the same time, as they did with Bosman.
But it does have a massive effect on their solvency if you're suddenly writing down/ off huge swathes of assets on the balance sheets. Exterior lenders and creditors would get a bit jumpy and borrowing rates for the clubs would increase significantly due to that reducing a clubs competitiveness and increasing outgoings in interest payments etc.
It'd favour the larger clubs who are hardly at any risk whatsoever of relegation (don't know if a similar clause would be added in for European football for them) so can maintain their balance sheet asset value.
Although write-down would be the prudent course of action it would only taken a few clubs taking the risk not to do so to effectively force the others to follow suit or risk being uncompetitive. Which then becomes a massive risk to the long term stability of the club if they get relegated because they'd take the massive balance sheet hit as players left and income dropped massively all in one go.