Nor did he intend to say much. He famously said "I don't write recipes for the kitchens of the future". He gave advice to organizers in his day, and left the future for those in the future, with the material conditions and relations of production they would have.
> there isn’t a proletariat any more
In the world of 1867, the proletariat was rather small. The world just shifted from majority rural to majority urban/suburban a few years ago. The proletariat is larger than it ever was. You can argue if it will ever be a vanguard revolutionary class or not, but it has not disappeared.
One point Marx and Engels made is the world has seen several economic systems over the past 10,000 years - primitive communism, slavery, feudalism and capitalism. The last three systems all collapsed under the weight of their own contradictions. Marx and Engels believed capitalism would run its course and eventually collapse under the weight of its own contradictions just like the previous economic systems. Capitalism has only become predominant relatively recently historically.