I think we need to slow down with the notion that we're going to have to give QB money to 6 D-lineman in order to keep them here.
Ioannidis just signed an extension at $7.25m per year which keeps him for now, 2 more seasons. When this contract expires he will be 29. We have Young completing his 1st year of a rookie deal, and Sweat completing his 2nd. That leaves (with the 5th year option) Young under 'control' for 4 more years and Sweat for 3. Allen and Settle will be entering their final year of their contract, while Payne appears to be but I believe we can pick up his 5th year option being as he was a first round pick.
I say all that to say this.
When Young and Sweat are due for their 'big payday' Ioannidis will have fulfilled his SECOND contract and will be approaching 30. We can sign Allen / Payne / Settle to 3 year contracts (similar to the Ioannidis deal) and get a whole 2nd contract completed before Young and Sweat are due for their big payday, so writing them off as 'unaffordable because of what Sweat and Young will get' is kind of irresponsible.
To add to that we have a very inexpensive offense with Roullier being the highest payed offensive player, not named Alex Smith, in regards to APY... followed by Morgan Moses. McLaurin has 2 more years, and he will get PAID, but by that time you've got Alex Smith and Morgan Moses off your books. It's possible you can move on from Collins at that point and those 3 contracts alone likely pay the whole of the interior D-line.
No I don't think we can keep all of our starting D-lineman for the entirety of their careers, but to believe that we can't get 3 - 4 highly effective years out of that unit and keep the window open for contention is not really accurate. I get that one of those pieces may be needed to leverage a trade for a starting QB, and it'll pain me to see any of them go, but I don't know that it's some foregone conclusion that we have some necessity to move on from one of them sooner because of the future cost is effecting a unit that is a major piece of our defense.
QB is imho, the most important position on the field, so the value is not overstated... but there is more than one way to skin a cat here, and I get the notion of going ALL IN for this limited window that we have cap leverage, as well as legitimate assets... but I also don't think there is only 1 way to make it all come together either.