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That's fine where the benefits of code compliance outweigh the costs of compliance.

But old homes are expensive to rewire. And often, the safety benefits are very small. Current code requires a lot more outlets than older code, in part because many people overuse extension cords and tiny/cheap power taps when there aren't sufficient outlets, which is a safety issue, but is it worth spending thousands of dollars to pull new wire through existing walls, and then repairing the walls afterwards?

Some issues, are worth retrofitting for, and hopefully a pre-sales inspection by the buyer or insurance inspection will catch those. It would be reasonable to have a transfer inspection for those too.