As has been said so many times before, "worth it" is very subjective.
Cruising vs. a land based are (perhaps just IMO) very different trips.
Biggest difference IMO is how you spend your days... the all inclusive nature of a cruise ends at the pier, and when you are talking about "seeing Europe", that isn't really done ON the ship unless you are really just satisfied with what you can see on deck. You do get the same room through the entire trip which is a huge plus for a lot of people as you only unpack once, and you get the dining on the ship as well which is again a plus or minus depending on how you want to see Europe. It is entirely possible to walk off the ship every day and not spend a penny beyond your fare if you want to just wander around, but most will go with a tour of some sort unless they're very independent and confident. That adds to the cost, but of course you'd be exceptionally hard pressed to add up to 20K. Folk also like sleeping while they move between locations, and of course the joy of sailing to begin with, so I'm not knocking that way of getting around. Most feel like they need to ALSO enjoy the ship, which takes time away from the ports. I've said it before and I'll say it again, for us personally, pulling out of port in Norway at 5-6pm every day ended up really irking us. We don't intend on ever sailing in Europe again until we are older and less mobile, if at all. We'd have much rather been able to have dinner, see a sunset, walk around more than get back on a ship we can easily sail on from the US. We LOVE sailing and I was blindsided by how differently I felt compared to other cruises we've been on.
The "value" of ABD is that you spend time in the countries the entire time. You still have someone else moving your luggage about (you do have to pack and unpack), and dealing with the logistics of getting you from place to place. You eat local food, speak with local guides, do local activities and the extra 20K is spent paying those folks, and to cover what amounts to included "excursions". The guides are like being concierge level (and I would argue the rooms themselves are an upgraded experience as well as they do choose very nice hotels) and the group size is infinitely smaller than your normal ship excursion size. Because you aren't tied to the sea, they tailor the trips to what is the best overall experience and not what is reachable from a port.