The OT site is effectively 'locked in' by a triangular shaped containment area.
A (straight) railway line on one side and a (meandering) canal on the other, both of which eventually almost converge as you head East (into central Manchester) hence the 'ground' itself therefore sits nearer to the apex of the restraining triangle than to its base (which is formed by) the West end /Stretford end and beyond that car parks W1,W2 and N3 etc. then acres of business/warehouse developments.
Ever since the early sixties various ideas for expanding the stadium meant development on three sides, West (Stretford end, a third tier), North (now SAF stand) extra tier, plus change of paddocks to seating and East end (use to be scoreboard end) again third tier added. the only change to South side was seating in paddock areas, and additional commercial development , restaurants, Munich Tunnel etc. all this limited by railway issues. It is rumored that the club has been buying up property on the opposite side of the railway (as houses came empty) so that some extension to the South stand could be achieved with modern building materials and heavy plant.
A new stadium would cost billions, not just to build, but to clear the surrounding land and 10,000 extra seats are not going to pay for that.
Glazers are making noises, but a new lick of paint, some screens and some standing areas are the best we can hope for!