I don't think Pokemon as a franchise should end yet, but the main series games are in dire need of a change of hands. Game Freak aren't capable of doing the franchise justice, and they haven't been capable for a long time now. They're farther behind the times than Nintendo are, and they're throttling the franchise with their wilful ignorance to the changes in the gaming industry. They've been releasing basically the same game since the first generation now and, whilst there is merit to be found in that in some respects, their stubborn refusal to build on improvements to gameplay that they make with each generation and then take out for the following generation has kept the series at what is now a painfully low standard for a modern JRPG. It's become much bigger than they are and it deserves to be in the hands of someone who actually knows what to do with it. Because they don't. They're one of the most inept studios out there, and if it wasn't for the brand name recognition Pokemon carries, I guarantee you that their games would barely sell. Game Freak are a HUGE problem. Their games aren't getting worse, they're staying the same, which makes them appear to be worse in a comparative sense. They didn't used to have the kind of competition back then that they have now, and their failure to acknowledge that they need to change is what is ruining the games. That needs to stop.
There seems to be this odd blind spot in criticism where Pokemon is concerned. All too often remakes are criticised for having the worst elements of the original titles in them, and yet Pokemon, which basically remakes Red and Blue every single generation, is excluded from this. People pine for the days of their favourite generation, yet fail to realise that the current generation is basically identical to their favourite generation. Pokemon never changes, which is both its greatest strength and its greatest weakness. There is a difference between changing and adapting, and Pokemon could definitely stand to adapt to the current times - the games especially, but also the franchise as a whole. There has been some effort to adapt with the release of smartphone titles, but this has come at the expense of spinoff titles, so I'm not sure I'd say it really counts.
I think it'd be better to say that Pokemon needs to slow the fuck down really. New generations are coming out too quickly, putting pressure on the already obsolete Game Freak to make new games, resulting in sub-par quality games, and an extremely cynical corporate attitude to what was previously a franchise that had a lot more care and attention given to it. Generations are getting smaller, and I don't think the "quality over quantity" argument holds when there is nothing but corporate greed stopping them from taking their time. What needs to end with the Pokemon franchise isn't the franchise itself, but the attitude and approach TPC as a whole take to it. Just because it has a reputation now and prints money doesn't mean that they should just put absolutely no effort into the quality of the products. Because there IS a decline in quality. There is no real effort made to adapt to the changing times, and this is a problem. There is definitely scope for the franchise to improve, but it won't, because TPC don't feel the need to improve it. The bare minimum effort will suffice, so that is what we get. That is what needs to end, because this uncaring corporate attitude is what will ultimately destroy the franchise, and has for several people already. They will bleed it dry and then unceremoniously discard it, which would not be necessary if they put the kind of love and attention into it that they did at the start.
Pokemon shouldn't end on the sole basis that it is still appealing to its target audience, though. In a lot of franchises there is this sense that it needs to evolve to suit its original target audience as they get older, but very few actually do that, and I think that's absolutely fine. There is merit in keeping some things the same: you appeal to the younger generation, and provide nostalgia for the older generation who grew up with the franchise. Pokemon still does this, and just because the older generation are tired of it doesn't mean they should stop - that just means they've outgrown the franchise. It's an incredibly selfish attitude to think an entire franchise should come to an end just because YOU are tired of it. So no, I don't think that is a valid line of reasoning...and this is coming from someone who is more disgusted with Pokemon than not these days.