What a complete load of delusional pish.
When was the last time St. Mirren or any other team like St. Mirren offered anybody a "bumper contract"? There's a reason for that.
Add ons/sell ons protect both the buying club and the selling club. They ensure that the bigger buying club doesn't have to pay the diddy selling club an unrealistic fee for a player who is largely unproven at any significant level. Whilst at the same time ensuring that if the player turns out to be a good as the selling club thinks they are, then they get rewarded properly in the future.
If Dundee Utd have made nothing on sell on clauses from Celtic, it's because the players in question have turned out to be pish. The only player I can think of who has moved from Utd to Celtic with any level of success is Armstrong and in his case, there is still every chance Utd could benefit from a sell on clause.