What I think you're running up against, particularly with the distaste for multiclassing you developed in 3e, is mostly a clash of styles. As long as a character has a lot of options, and 3e certainly added a lot from feats to skills and level-based wealth driving magic equipment, some types of players were going to map it all out with a plan to maximize their chances of getting the character they want. And that's even without talking about multiclassing and prestige classes. That player may not be you, they may even drive you crazy, but their way of approaching things is as valid as yours.
I agree with your observations on players and playstyles, but I don't agree that 3E provided a lot of options. IMO 3E offered the illusion of options but most were either unavailable or you took a severe penalty for exercising them so they were not really viable options.
I completely agree that there are characters who map out their 5E characters for levels 1-20 at game start (I even do this myself at times). But it differs from 3E in two important ways IME:
1. Plans are not permanent in 5E. They are plans. As I noted I have mapped out some characters, but the characters I have actually played "as planned" are very few. The mapping in 5E is a plan which you can (and in my case do) easily deviate from without a lot of bad ramifications. In 3E your plans were a binding pact. I remember building a Swashbuckler-Wizard-Bladesinger in 3E. This guy was going to be awesome getting his intelligence bonus to damage on finesse weapons with some nice spell options and a good AC, white roomed it to death and then we ended up playing in a campaign with a crap ton of undead who were immune to sneak attack and therefore her damage bonus. Problem was by the time I figured this out there was not a lot I could do to salvage it.
2. In 5e there is not a huge power disparity between the players who map and the players who don't. The guy who just picks his level up on the fly is not going to be that far behind the guy who planned it out from the get go to squeeze the most out of things. In fact in 5E I would argue the disparities in ability scores from good dice vs bad dice at character creation cause bigger disparities in power than disparities due to one player planning out a great multiclass combo and the other guy just hitting the level up button and taking an ASI.