Surely an opportunity cost associated with those two hours a week. Say if the player instead spent that time hammering the ball towards goal with his stronger foot to improve his shooting? Which would improve the player more overall? I don't think it's as simple as saying becoming two footed is better.
I also think there's some biological factors involved. May be genetic, may be just what's learned as a kid, but by the time a player is in his mid or late teens I think they will respond differently to attempts at improving their weaker foot. Some might do great like Best, but others (like Bale perhaps?) will not do that great. I don't think it's simply a question of effort or time spent working on it, different people will see their bodies respond differently to the same amount of practice. Then you get stories about those "who just worked on it" and got better, but those will most likely be those that had the predisposition to get better quite quickly.
I also think comparisons with lower level football aren't particularly transferable to PL level/top level football. The level you have to reach to effectively use your weaker foot in the PL is a lot higher and you have to get a lot further along the learning curve for it to pay off than at a lower level. The level of accuracy and consistency required is so high that getting "comfortable" is a much bigger ask than at lower levels.
(Not to say that for some players it's probably been down to effort and/or poor decision making in part by their coaches and in part by the player)
