... There is already alot to spend exp on and death is right around the corner. These power builds will die just as fastbefore they ever get made.
Some GMs (such as myself) let the players in the group create new characters with the same amount of experience that their lost characters had accumulated.
The reason I wouldn't apply a rule like this to my campaign is that I think most games that use professions/classes are intentionally designed around not letting individual characters be too self-sufficient. Because of that, the players' focus should usually shift towards building a strong party rather than characters that don't really need each other for anything.
Additionally, many of these games' pre-built challenges explore this very facet - so if a GM relies heavily on this sort of material, some effort on re-balancing challenges would be required.
But if this is something you and your group are looking for, then absolutely! In that case, I would just recommend making the cost of "multiclassing" higher than what you've originally proposed, for balance reasons. Or you could maybe port the Forbidden Lands setting into a skill-based system such as Genesys (FFG) or one of the BRP-based games - Mythras and RuneQuest are great options.
Sure, the cost could potentially use some adjustments. Maybe cost it like skills? Rank x5.
Either way. It balances out because someone who does not invest in the talents of other professions will have more skills (and thus more dice to throw) or more talents from their own profession/general talents. Someone who invests in other class talents spend more to get less.
Additionally, the general talents already let everyone get some kind of perk in everything. It's just that the more interesting talents are in the profession ones. Nobody is going to be the best at everything because the increased cost is going to be prohibitive.