Annoyingly there have been several 2 parters in the new series that have seemed padded, and just as many single episodes that could have done with being a bit longer. Plenty of the one hour specials have seemed overlong IMO, so I think 75 mins may be a good compromise, but until we see it/know the story who can tell, it might still feel padded at 75 minutes for all we know currently.
You may well be right. And, ultimately, if its an enjoyable story that fits the length well, I won't be complaining afterwards!
But, I do think that a story that fits 90 minutes well can be a bit more epic. And, we need epic for the 50th!