I see no reason to prohibit people that wish to spend TP on hires from solo'ing a quest with 4 gold seals and a regular hire and getting the bonus, that is more sales for the game and not really going to hurt anything. Those that wish to spend TP to get the higher bonus, let them. I also see no reason why shortmanning a quest with three hires and three players would matter either, it certainly isn't going to hurt the game if people want to run in selective groups or with family.
Seriously, dungeon scaling affects me so little, that it is not even an issue for me. I still complete pretty much every quest I start either in a full group of PC's, solo, solo with hire and solo with several hires. Dungeon scaling is such a minor thing really that I don't honestly see where it even needs to be brought up, worried about it? Don't group and go with the standard Estreak bonus if you like.
The problem with BB is that running the quests solo is much simpler for us experienced players, we don't have to worry about losing that 10% exp(5% really). Make give me even more experience to open my solo TR's leveling to all players, and I won't mind filling with new/inexperienced players, I am pretty sure that up until about level 13 or so I can mostly carry them if needed, not a real big deal, and guaranteed I can carry 5 people until level 8 for sure. By then maybe I would have met few good new players too, and the way it is right now, I don't even open LFM's until pre-teen content, and that is only for a few quests all the way to TR time.
Just a means to incentivise grouping to make it more valuable than not grouping is. And seriously, there is no real reason to punish anyone that uses hires, whether Gold seal or not, they are harmless and really a non issue, and might make Turbine a few more bucks in an honest way.