I agree the only way he extends is if he gets some big time stability on the back end, rather than sign for a year or two and then try to cash in big time as a ~32ish year old catcher.
I also think that I would sign him to that type of deal if he's open to it. It's certainly a big risk, but if he hits like we hope he will, he'll be worth several times what he'll be paid for the early part of the deal and still useful even if he's not worth every penny as a 1B or DH if he has to move off of catcher for the last couple years.
If he struggles and ends up not being worth anywhere near the contract, well then we were probably going to be bad anyways, so we're not really any worse off overall. Also, unless we extend Jones or sign a big FA, we aren't going to have (m)any guys making big time money in the middle and later years of an 8-year contract for Wieters. Nobody else on the team still in the service time area really seems like we'll need to lock them up to big time contracts. Maybe one or two of the young starters...maybe, and they still don't project as #1 type big time money makers.
I mostly agree with what you're saying, but I don't care for the line of thinking in bold. Yes, he will be worth much more than his pay in the first 3 years, but that's not really an added benefit since he's already controlled for those years anyway at a likely reasonable cost. To me, the part of the contract that needs to be thoroughly examined is the 5/75 part. If you (general you) don't think he'll be worth more than 75M over those 5 years, then you shouldn't be for that contract.
So 75M for 5 years (age 30-34 seasons). That's a tough call for me. If he doesn't have any awful injuries, he should still be at least a near AS C for the first 2-3 years. The last 2-3 years is largely dependent on how well he will age and if he becomes good enough with the bat to be worth a fair amount while playing less at C. I think if reasonably healthy, he should be good enough and age well enough to be worth that contract, but it's such a big risk to give a catcher going into his age 27 season a 8 year contract.