I think this is a great question, and really points up both the plusses and minuses of cruising.
First, I think you're smart to want to get away from the Caribbean. The world is too big to keep going back to the same islands time after time.
Of course you're absolutely right that the ports along the Mediterranean are historically signficant. It's no exaggeration to say that in many ways they are the cradle of civilization. For those whose shore excursions have been limited to beach trips, shopping sprees and rum punch, getting your brain around the idea of exploring the amphitheatre in Ephesus where Paul preached can be a bit daunting.
You're also right that some cities are so far from the port that much of your valuable port time is taken up with pure transportation. Rome, Paris, London and Berlin are examples of these.
So, I think there are two ways to think about the issue:
First way. Take the cruise, but approach the big cities as "special introductory offers." In other words, have the mindset that there's no way you're going to "do" Rome in a few hours. Instead, change the goal to one of seeing enough to let you decide whether this is a place you want to come back and see later on a land tour. The smaller stops, like the Greek Islands, you can probably do in a day and be happy.
Second way: Cruise to Tahiti and plan land trips to Europe and the rest of the world separately. Just be sure you're not kidding yourselves about the land trips. If you're 40-ish and haven't done it yet, well, maybe it's time to make specific plans. If you can't bring yourselves to do that, revert to "first way" above.
For our part, we mostly did it the second way, more by accident than design. We took many trips abroad for both business and pleasure early in our marriage, and until the last few years we took far more land trips than cruises. Even now, we almost always manage to extend our cruises with significant land visits at one or both ends (easier to do now that we're retired). We almost feel cheated if we can't do a substantial pre or post-cruise stay somewhere wonderful.
But when we're on a cruise and hit the occasional new port we revert to the "first way," and enjoy it while deciding if we'll put it on the list for a return visit "sans bateau."
Hope this helps.