I'm not sure he could get a better deal elsewhere given his fitness issues, even if you take into account free agents signing on fees.
I think the fact of the matter is that his signing is largely viewed as a failure by fans of other clubs, and even by lots of our own, and that is 99% down to his fitness issues. He has shown that when he gets a run of games he can be very good, if not quite the game changing midfield signing we expected, but his form always takes a massive dip after an injury which outside of last season was very often.
He also gets a ridiculous level of backing from both Klopp and the other staff, Ljinders really bigged him up in some of the recent quotes from his book, and seems like he is well liked within the squad.
I think an extension based on slightly improved terms with some appearance based clauses will probably be the way they end up going with Naby.