What is the point in comparing 90's attendances when everyone got fewer through the gates by a huge amount?
Hypothetically, if there was no limit to the size of the ground, no restricted views. No issues for day trippers with no memberships or lack of 500 loyalty points. No "single seats only remaining". We'd have averaged at least 35k in our first season back in the prem. Every game (save for the Bournemouth one in 60mph winds) was effectively sold out if you discount single seats and restricted views.
It's not even far off in the championship. 3 sides of the ground are effectively (same issues permitting) sold out nigh on every week. Only the Bramall Lane Upper is available for anyone wanting to turn up on the day and buy 2 or 3 tickets sat together. Of course this is only possible if you've a purchase history. Without such restrictions we'd be even closer to selling out games completely.
We've sold over 22k season tickets again this season. Considering we normally average around 6,000 more across a season than the number of ST's sold, we're looking at a 28k average in the 2nd tier. Which is now only around 2k lower than our realistic capacity. The ground NEEDS expanding if we are to grow as a club.
All that said, it is not the priority. The squad, the training ground come first, but it is on the list of things required if we want to grow. The fan base will never grow if they can't get to games.