Some experienced cruisers fetishize the idea that a local tour is always better than a ship tour, even total costs aside. There is some value to the cruise's experience and size, for example in skipping (possibly) some lines, or getting in to an occasionally exclusive location. (I don't discount that luxury private guides have their own exclusive values.)
There are some cruise ports where distance to the sights is a major factor. Some typical examples are Rome, Berlin, Beijing, Granada, and (now), Venice. It is SO FAR to the place you're seeing that both efficiency and not-being-left-behind by the ship are worth something. We went on a cruise that stopped in Busan, Korea and offered an excursion (involving a high-speed train) to Seoul, which was so crazy an idea that we settled for the inferior destination of Busan.
Even on "river cruises" (I mean, where the port is at least in the same city as you want to see), it can be necessary to take a shuttle bus to a subway station to take a train to explore on your own. Be careful what you wish for.
I am not bad-mouthing cruises at all. We've been on eight ocean cruises and two river cruises. They have their values, especially for people who don't go to Europe frequently, or don't wish to spend time planning an independent trip.
Edit: I remembered two cruise war stories:
We wanted to go to a major seasonal music festival event on our own, overnight port. Because we paid the SHIP Cruise Director to buy our custom tickets and hire a round-trip taxi for our one-off excursion (hundreds of dollars), it turned out that he had given his personal phone # to the taxi driver. (Cruise Director spoke the local language.) On the way back from the event, it developed that the ship had been ordered by the port authorities to MOVE to another pier, on the opposite side of the port area! The cab driver (who spoke virtually no English) FOUND OUT and delivered us directly to the ship!
We booked a very distant SHIP excursion from one of two Honduras ports to Copan, an ancient city. The trip was great, even including a superb hot lunch. The trip back (3 busses, AFAIR) took much longer than the trip out (over two hours, in any case) and not only were we late, but the ship was forced to move miles away, because it was departure time! The guides on the busses found out where the ship re-docked (!) and took us there. I also think they held dinner for 3 busses worth.
(It may have sounded unkind that I used the word "fetishize", but this is why I chose an extreme word.)