Marx presented the Bourgeoisie/Proletariat dichotomy as something that's emerging.

"Society as a whole is more and more splitting up into two great hostile camps, into two great classes directly facing each other — Bourgeoisie and Proletariat."

Notice the use of the present tense. The peasantry was still being shaped into the Proletariat. It hadn't vanished completely. Same with the nobility, the clergy, etc.

Later Marxists, especially those in developing countries, continued to define the peasants as a separate class. Even a century after Marx, Capitalism hadn't fully solidified the two class system.

Marx's "two camps" lives on, even when "two great classes" is not an accurate reflection of reality. It's not uncommon for peasants to join the proletarian camp politically. In the developing nations, even the bourgeoisie can be part of the proletarian camp.

Given that the two class system was always a prediction of the future, and never an established reality, we don't have to dogmatically cling to it in our analyses. It's possible to identify classes and subclasses in developed nations, as it was in the semi-feudal nations.