.NET is stable, they very rarely remove things. Code written for .NET Framework 2.x will probably work fine on .NET 8 (unless it uses the things that didn't make the jump, like WCF, but there are libraries that provide SOAP support.)
Can a single developer remember all of .NET? No, but that applies to most languages/ecosystems. But if you encounter something you've never seen, you can always check the docs.
And the thing hasn't been named ".NET Core" for 4 consecutive releases now.
The mixture of client-side and server-side code that Blazor can allow in the same files gives me a very gut instinct feeling that "those who have forgotten the WebForms history, or worse the ASP Classic history, are doomed to repeat it". It's obviously different from both WebForms and ASP Classic and its use of Wasm is certainly a brand new idea that WebForms only wishes it had access to. It seems like a brand new mistake to me, because I still remember debugging ASP Classic and WebForms and Silverlight and "now even the client side JS of ~~WebForms~~ Blazor is no longer JS but a mini-.NET runtime in a Wasm box" doesn't sound like a good time in the long term maintenance future to me when Blazor apps become legacy apps. But some of that gut instinct may just be cynicism at this point.
Can a single developer remember all of .NET? No, but that applies to most languages/ecosystems. But if you encounter something you've never seen, you can always check the docs.
And the thing hasn't been named ".NET Core" for 4 consecutive releases now.