So basically the OPs argument can be summed up as such: "The games are mediocre because they come out too often and repeat too many elements. I might be interested if they retire them and replace them with new franchises." And that's compeletely fine, but your reasoning is very subjective, which is why few people are taking you seriously.
The introduction of new characters and story can add tremendously to the feeling of freshness of a new game. However, at the end of the day, these are games. New characters and story actually do nothing to change the gameplay unless such gameplay is directly related to the story - and it isn't always. Many studios create new IPs that have interesting stories and charcters but do very little in the way of gameplay innovation. Some new IPs are very innovative. Yet regardless of gameplay, they are deemed original by default. The ones that have innovative gameplay, are, of course, looked at more favorably, but studios get seem to brownie points for simply making a game with a different name.
Take, for example, the bunch of action games that come out of the industry. Companies make action games with the same stealth mechanics, the same third-person shooter mechanics, the same parkour mechanics, etc. and yet it's supposedly something completely new. And Final Fantasy. I love that franchise too - it's among my favorites. But how does varying casts and stories make the gameplay any more original? Standard turn-based for 1-3 and 10; ATB for all the other main ones. Variations on how you level up and gain abilities. Treasure-hunting, occasional boss fights, missions, weapon customization... Sounds like a pretty run-of the-mill JRPG to me. Yet while NIntendo games do introduce new mechanics , art styles, etc. that alter gameplay, sometimes more effectively than other companies in the industry, you automatically dismiss them because they they have Mario and LInk in them.
Why do you care so much about characters and story? It's a game, not a movie. You can get as much from Let's Play for the same amount you charge for Internet.
And while you may want them to foergo these franchises and replace them with new ones, the statistics make such a suggestion nonsencial. As has already been pointed out to you, Nintendo found success with these franchises in numerous different generations. They continue to be well-received by both critics and fans, and continue to sell millions. They certainly have their titles that didn't do so well, but if you look at the circumstances, they're on consoles that didn't do so well. If anything, there's more an issue of making consoles that people want and proving a variety of different games (whether through first- or third-party), not a fundamental decline in core franchise interest. I think most everyone - NIntendo fans and otherwise - would agree that NIntendo needs to focus less on core franchises and make more new IPs. But even then, it isn't all that simple. New franchises, with a few exceptions, don't just underperform, they do bad (all the ones that do well are casual). Do the research and you'll find a ton of new IPs in the GameCube and Wii eras that did terribly. That isn't to say, of course, that they should make new IPs, they just have to make sure their concepts for new IPs are really good and they appriopriately balance investments with old IPs. And this, again, is the general consensus.
I can understand your fatigue with Mario and Zelda. But get rid of them entirely because you can't stand them anymore, when so may others love them? What makes you that special?
And you have a lot of misconceptions about NIntendo games. You seem to be one of those whose attitude towards NIntendo games is "you've played one, you've played them all" yet you can't even fairly make that judgement if you haven't played them all. Sure, people make mistakes, but it's hard to think you know what you're talking about when you say "Zilda". The Wonderful 101 is an RTS, not a platformer. The mechanics of Pokemon have evolved so drastically that it's hardly the same game anymore. If you think all of the following changes/ additions since the original games: of breeding, EVS, IVs, happines, ribbons, double battles, triple battles, sky battles, rotation battles, new abilities every gen, mega evolutions, weather, the physical/special split, new moves and teaks to those moves' effects, contests, battle facilities, strolling with pokemon, variations on safaris, three new types, and over 550 new pokmon (whether you like them or not) didn't change the game at all, then I have no idea what to say to you. Zelda nd Mario may have similar structures, but they've changed a lot over different installments. Different levels, different worlds, control variations, art style, different items/power-ups have made pretty big differences in how you play. While I agree that they re-use way too may assets (New playable characters in Mario? Why are almost all the locations in Zelda the same? Why 8 gyms with linear connecting paths?), to say that the games are all the same is not just opinion - it's simply false.
I'm not denying that Nintendo has a lot of problems, but the notion that their core franchises have declined in quality mainly because they've existed this long has very little basis in reality. And even if they have declined in quality, that amount is far from substantial. What's far more likely is that is that your interest has declined - to which I acknowledge that you're definitely not alone, but you're not going to taken seriously if you use that as a basis for arguing that the games are bad.