True, the budget of the top English clubs is already considerably lower than that of the 2 Spanish giants, mainly because of the TV-revenue in Spain that's not equally spread among the other Spanish clubs.
Look, why don't u move to the US Coys? Here every club shares their revenue and there's a salary cap. The PL already share tv revenue pretty evenly, if they share more sources of revenue without the rest of Europe following the trend, it'll put a lot of clubs to a dissavantage towards the rest of Europe.
From ManUtd's point of view, it would be quite stupid to also begin to share the commercial revenue, when they already have a considerable disadvantage compared to Spain.
For the TV-revenue, you still can debate that it should be distributed equally (like it's being done in England currently), because without all the other clubs, there wouldn't be any exciting matches for the viewers to watch. However, I don't see how it would be reasonable to share the commercial revenue as well.