Backstory schmackstory. Nobody gives a crap about what happened to your character before you started play.
Please keep your One True Wayism to yourself. You might not care, but I do, and lots of others do as well. Hells! We have entire mechanics (background) that rely on just that now! I happen to be one of those people that think that the story of each class should matter. So, yes, I very much take into background as well as curent story issues with what player characters can learn.
5e assumes that, if you're playing a pure wizard, you're not going to be putting a 12 or lower in INT when the book is suggesting it as one of your big attributes. That's kinda silly and really kind of a strawman, something to try and hit and beat like a dead horse when, for the most part? Its not going to be coming up outside of a roll-for-stats game getting some seriously bad rolls and has to keep them. This 12 and below INT wizard is nothing more than white-room theorycraft that is just not going to show up in the overwhelming majority of games. Realistically speaking, the only time I ever see it show up is when someone is planning on multiclassing and is dumping INT as much as they can.
Multi-classing rules don't exist for the sake of people who want a career change, or if the players want a hybrid character. That's not what they acomplish. Hells, even in previous games, that's not what they did. Hybridization is acomplished through subclasses now - that's why there's the bladesinger, the eldritch knight, the divine soul, etc. There are no retraining rules, like in previous editions. What the MC rules in 5e are for? Level dipping. You take 1-3 levels of one class to enhance the abilities of a second so you can be better at said second's. There's no "dropping" being a wizard because you fully intend on keeping all those wizard abilities and using them in the second class. The example about swapping from engineering to sales is nothing more than a red herring, because its ignoring that, for this comparison to hold true, you'd somehow need to use engineering skills to work with your sales skills. Generally speaking, engineering and sales don't use the same skills sets, while a wizard-turned-rogue will still be using wizard skills in their "job."
I feel like you've started with the position of liking attribute minimums as written, and worked backwards to try to justify it.
Glass houses, throwing stones. Same thing to you, but adding "dis-" in front of "liking."