There is a logical basis.
We have, as you say, a ST waiting list. That is for a stadium with the poorest facilities in the league. Very often the only spare seats at GP are for single, obstructed views and they still sell so imagine a decent view, first rate facilities and the chance to buy ticket(s) next to family or friends. Then there is the new stadium bounce that every single club that has moved grounds has experienced. Add in the most accessible stadium in our history within walking distance of town, with a dedicated Merseyrail station and city centre connections.
Let's be pessimistic. 40k now plus half the ST waiting list and we're at 44k. Add in an additional 3k (low estimate corp) we're at 47k. Let's say 3k turn up just cos it's dead easy to get to the ground and you're at 50k without barely trying. Those estimates are all low ones and we haven't even talked about decent pricing structures and the usual new stadium bounce (which is quite large on average if I recall).
Add in the history we have of 60k+ and even 70k+ we should never be settling for 50k. It serves zero purpose.