Not an answer, but it was too long for a comment.
Even if the toilet was good for 100 years, I don't think the salesman would necessarily say so. A lot can happen in 100 years that breaks the toilet, without it being the toilet's fault, but people blame the manufacturer anyway. Whenever you provide extreme warranties, you have to be prepared to deal with extreme claims, which may not be worth it even if your product is good.
For sales purposes, I doubt many people will think much of 10 years vs. 20 or even 100 years implied warranty. You obviously care, but then would you be willing to pay even double for a toilet that claims it will last 100 years vs. 10? A decade is "long enough" for the purpose of making a sale, after that you get diminishing returns.
Also, a lot of people don't live in the same house that long. This group is probably correlated with the most avid redecorators (like flippers). Someone who is expecting to move in 5 doesn't care that much about a 100 year toilet or 10 year - when selling the house, how will you convince the buyer to care about the expensive toilet you bought? And even if they did care, is the company even around that far in the future?
Generally, I would be surprised if modern toilets lasted longer than older toilets. Engineering has advanced a lot. I define engineering as "doing more with less" or "smaller error margins", meaning the modern toilets might be less overbuilt than the older ones. Finding new ways to cut corners on materials of old goods is one of the main ways in which the economic growth happens after all. That said, a porcelain bowl is not that fragile and if not abused it should last many decades. That's actually where I'd expect a difference: older toilets can probably survive the same time despite abuse. When I say this I'm referring purely to things like the bowl cracking.
You could also have substances stick to the inside of the channel and impede water flow. To be fair, these can be cleaned to some extent by a plumber. The design of the toilet could affect how quickly the accumulation happens. This is where a modern toilet can outperform older ones with better design, or underperform because they overestimated the effectiveness of their particular design. You're stuck rolling the dice.
There are moving parts, seals and gaskets in a toilet as well. These will need to be replaced after a few years as they degrade over time. But that's not really part of the toilet.