Ironically, Senna had 27 on his McLaren when he won the 1990 title because Prost had taken the number 1 with him to Ferrari after winning the 89 title with McLaren. One of the vagaries of the old system was that the numbers went with the team except for #1 which went with the champion (and thus #2 went with his team mate).
Few drivers were associated with a number prior to the fairly recent change to allow them to run a chosen number, so retiring a number makes little sense because, in reality, many of the low numbers have been used by many drivers over the years. Latifi uses 6. If he leaves F1, do we retire the number? Why? Keke Rosberg used 6 in 1982 when he won the title so perhaps it's more Keke's number than Latifi's.
And if one decides to only retire ~"special" numbers, how do you define "special"? Multiple titles e.g. Lewis? Dying during a race e.g. Jules? Perhaps just longevity e.g. say Latifi does another 10 seasons, do we then retire #6 as it's "his" number?
Go back to the old days, and the driver's numbers changed from race to race. In 1963, for example, Jim Clark had the numbers 1, 3, 4, 6, 8, 18 during the season. Clark is definitely more notable than, for example, Ricciardo (#3), Norris (#4), Latifi (#6), Stroll (#18). Even the winner of the previous year's title didn't get to keep his number 1 through the season - G. Hill had won the year before (1962), for example, and was only allocated #1 three or four times in 1963.
The only reason for having consistent numbers these days is the HALO. It prevents the driver's distinctive helmet being easily seen by spectators - back in the day spectators didn't need numbers because the helmets were all individual. You could tell Senna from Prost in the McLaren, for example, by the helmet alone.
Turbo says "Dumpster sounds so much more classy. It's the diamond of the cesspools." oh, and "The Dutch fans are drunk. Maybe"