I've owned, and operated, lawnmowers for decades. During that time, I've worked on many of them, and the repairs included just about everything, including rebuilding an engine. Over time, I've noticed the newer engines last longer than the older engines. Whether it's because they're built better, or the newer oils are much better, is something I don't know, but in the past, the engines would usually present problems long before the decks did.
These days, the decks on riders just don't seem to last that long; especially if they're stamped. Replacing them seems like a good idea, until you realize the cost will probably be over a thousand dollars after you buy the accessories that need changing too, and you perform your own labor. Depending on the model, that can be at that magical point where it's so close to half the price of a new one, replacing it is a better option.
I gave a way a fine mower a few years ago. I'd changed the deck once, but the new deck only lasted three years. Changing the deck was still an option, but the mower was old, and I foresaw other things going out, with the knowledge the stamped deck would become a problem. Still, it was frustrating, and only showed that to get long years out of a deck, paying for the most expensive riders, with a heavy welded deck, was the best option. Still, they don't last forever, but if you have a welding machine, building a new one is always an option. Well maintained, the engine will last a long, long time, and replacing belts, or pulleys is usually an easy task.