This is what you said initially:
Plus, potential investors will be aware of the value of a huge plot of land on the edge of the city center (in terms of housing).
Then you changed it to this:
The argument could be made that if we "want to compete with the very best" we need a 50k stadium
That's two completely different arguments. The first is a stick to beat a new owner with before he's even arrived. The second is a bit of an odd criticism. A lot of people are calling out for serious investment and I think most people accept that will only come from a new owner/investor. What you're describing is a natural part of any owner's ambition - a stadium with increased capacity. McCabe has had plans drawn up to increase capacity.
For it to get to the point where we would be looking at a new stadium because the Lane isn't big enough would require a prolonged period of PL success - where the ground is full for most if not all games and there is evidence that thousands more would attend if they could. I'm as hopeful as anyone, but I don't think that'll happen for a while.
If it really does get to that stage, I think the owner would still consider staying, because a city centre club is something special, which owners want, and because unlike other clubs that have moved, I think the % of fans supporting the move would be very low. Unless things change dramatically in Sheffield, the real estate value of the Lane won't be that interesting for developers. I can see massive costs and planning difficulties moving.
I am completely with you when you say that would be the end for you if we moved. I can put up with almost anything, but that would be it. The club is Bramall Lane and the Lane is the club. However, I think we are fortunate that it is already a good ground in a good location, with decent capacity and potential for expansion.