Cruise lines often retire their cruise ships as they acquire new ones or commit their operating budget to more modern vessels, ushering in a whole new era in comfort and service. The cycle isn't exactly a set one, especially following the disruption of 2020.
While occasionally ships end up scrapped, with great frequency they're refurbished by new and innovative cruise lines. It'll be interesting to see where some of these end up as we look to a Better Future in Cruise & Travel.
Roughly 13 ships were retired from Carnival's fleet during 2020, alongside a handful from operators I don't typically engage such as Marella or Costa.
However, we'll be sad to say 'so long' to a few more memorable experiences such as:

Royal Caribbean's Majesty of the Seas and Empress of the Seas will join the previously scheduled retirement of the Grandeur of the Seas later this year.
Holland America had been planning the retirement of its oldest ship, the MS Maasdam later this year.
It seems as though the Celestyal Olympia will be retired as well, not long after an exciting replacement arrives to Celestyal's destination-specific fleet, the Celestyal Experience.