Automatic Transmission Leak – How Much Damage Caused?

My automatic transmission broke down at end of long journey, roadside repairman found transmission fluid had run dry, he put 3 litres in to get it moving so it could be taken to a repair garage. Garage told me it’s got a fluid leak from ‘front main seal’ which they can replace – BUT they warned me this may not fix the problem and I may need whole new gearbox ? (because car was driven with gearbox fluid drained dry) – they said this would be expensive, and as car is over 20 years old, it might not be worth it? Car has been taken out of use to prevent further damage, but it still drives – there’s a delayed engagement of gears, the engine revs high but car remains stationary for few moments before eventually lurching forward. Would just a seal replacement fix the problem, or is it likely that the whole gearbox will need replacing? would really appreciate any advice – thanks.