Those options are in the corebook. I don't think it's unreasonable to ask that the designers took basic core book options into account
They're in the core book, but they are cited as options. They do advise that playing with Feats and Multiclassing will affect how the game functions. So I think they did take these into account in that sense...they explained how using those options will change some of the other base assumptions of the game.
That is them saying "these things will affect your game, so if you allow them, you may have to adjust for them".
What they also provided in the core books are options for monsters and NPCs; beside the Legendary Creature options, the DMG talks about modifying monsters and creating new monsters. The DM should use these options to help achieve the results he wants for the party he has.
My players are experienced players, but they are not necessarily min/max types. They make effective characters because they understand the way the game works, bu they are also mindful of character concept and the fiction of the game, so combat efficacy is not their sole focus. So therefore, they don't build their characters only around how to deal as much damage as possible. This does not make them ineffective players, or supoptimal, or any other label that people care to throw about. They are perfectly capable of building characters that are nothing but murder machines, with every bit of to hit bonus and damage output maximized and with little regard for anything else.
They just realize that playing that way actually winds up making the game less fun. Like playing basketball with your 6 year old brother when you're 18. Or, if the game is not easier, then it's because the DM had to adjust things for their style. They know that the fun of the game comes from challenge.....so they don't do everything in their ability to reduce the challenge.
Now, this sounds like I'm criticizing any playstyle other than my group's....but really, I'm not. For some folks, making the most combat effective character possible is the fun of the game. And that's fine....good for them. But DMs for such players need to then do the same with monsters. They need to make them as effective as possible. Otherwise, there's an imbalance, and it will show in play.
And this is my point...whatever play style or experience level the players may have, the DM should be able to challenge them. He may have to have the monsters use tactics, he may have to increase or alter their abilities from time to time, he may have to send higher threat monsters at them....but he can challenge them.