...yet seat covers are seat covers...
Not here, not on older Mercedes I'm afraid. Much of the value in collectible MBs of this era is in the fact that those who maintain them usually adhere to trying to keep this somewhat original and of reasonable high quality. A number of folks here wait to do it right, rather than do it now with an inexpensive solution. That's those usual vendors like Heritage, GAHH, German Auto Tops, and a number of others catering to these cars. A thrifty redo of the seat covers, and then not matching the door panels, the dash, and even the little inserts on the window winders and ashtray will mark your car as, well, a thrifty redo. Now, there's nothing wrong with that as long as you understand it and it suits your needs. If you ever go to sell your car, with a mismatched interior or one of not so great quality, it will not command the kind of prices that go along with one properly done. If you don't care to ever sell, then it's OK.
At least with seat covers, they can always be pulled off and replaced...
The boat analogy isn't exactly the same kind of deal. There's no such thing as a collector pontoon! People don't strive for originality. Now, if you had a $200K Cigarette boat (of which there are a number on Lake Norman) with a bespoke interior, and a seat got destroyed, you'd probably want the repair or replacement it to match the rest of the boat's interior, yes? You wouldn't just take any old seat cover; you wouldn't go to some guy in a garage in Mooresville...with all due respect to my friends in Mooresville! You'd want it done properly, like they did at the factory.