You sound kind of butthurt over this (no offense)
I'm not saying every senju is a prodigy to behold just like not every Uchiha is either. Even if it's fodder Senju fighting fodder Uchiha, apparently those fodder uchiha didn't have what it took to take down the fodder senju. You make it sound like the Uchiha were utterly and completely stompped by the senju. This is not the case. They were tired of the feud and , more importantly, tired of fighting Hashirama.
They opted for the peace Hashirama offedered. People misinterperate this as defeat and that isn't the case. If anything they both fought to a stalemate fighting any longer than they did would have just spelled the end of both clans.
What makes the Uchiha dangerous are the ablities you described earlier. But just because they copied a jutsu doesn't mean they are going to be better at it than the one they copied it from (and why would they? Sure they copied it but never trained in how to use it effectively)
The Uchiha have powerful jutsu yes
The Senju are remarkably durable
That means they can take more punishment than your standard uchiha. This is what made them such a threat to the Uchiha. Try to think of it in terms of endurance. Both can constantly keep tossing jutsu at each other one after the other, but unless the Uchiha can overpower the Senju he/she is fighting, the senju will outlast the Uchiha.
This is just your standard Uchiha/Senju clan member. When you get those who use MS of course Senju start dying out and a more skilled Senju is required to fight.
You mention Izanagi but they could only use that once or twice, after that they loose their sight for good. If they don't kill their opponent after using it, then they are as good as dead.
This really doesn't seem like such an inconstant problem to me. But then I am not looking at it the same way you are I suppose.
No worries. You don't have to go through all that work. Your explanation is sound imo so if I am indeed mistaken I have no issues admitting my fallacy. Your logic seems legit here.