Yep fully agree with your assessment. To justify having a 48k stadium as appose to a 42k seater stadium (the extension), you don't need to be selling out 48k every week. In fact if you are selling out every week it indicates the ground is too small.A top half Prem Sunderland could attract 45k with 3k aways. A Sunderland with a big takeover and Champs league ambition (yep, never gonna happen), would see us need an extension to 55k. The reality is we will probably at best become a bottom half prem side at some point, with potential for crowds of around 40-47 depending on the occasion. The extension was definitely needed. If it remained 42k, thats 39k SAFC at the time. Not enough.
It's realistic that we can become a mid table premier league team. If we do then most weeks we'd be getting 43-44k. On 3/4 occasions per season we'd sell out. The ground would be very much the correct size, nobody would be having this debate.
When the ground was extended at the turn of the millennium, our expectations could and should have been a lot higher than making up the numbers in the premier League. The landscape was very different then, we had momentum and should have really kicked on and established ourselves as a strong top 6/7 team. That ship has sailed.
The notion that the ground is too big for the club is as a result of underachieving for a sustained period, then the club going through it's worst period in its history. For reasonably well ran Sunderland a 48k seater stadium is just fine.