When did it become a standard to earn many grades vs. learning one very, very well?

I can't answer this question. I was lucky in that my sensei spoke often of learning our system very, very well before doing anything else. I can say I have one particular flaw in that I did familiarize myself with other systems but stayed with my "one system" for these past 40+ years, i.e. since 1976. Even today I find interesting things to do with my practice and I can say that often it is the familiarization of other systems and the way they do things that have inspired me in my practice. My system is still the same but different in interpretation today vs. 1979.

I have no idea why it become vogue to suddenly require more and more credentials, i.e. black belts in many systems, where the resume' tend to seem impossible. I, my view here, spent a considerable amount of time trying to learn my one system adequately let alone trying to learn many systems with many requirements different than my system in a time span that seems impossible. 

Most of those guys who do have a "few" additional credentials tend to have a few, i.e. credentials in say karate, jujutsu, and kobudo for instance. Some well rounded knowledge that covers the stand up striking to the grappling to the ground work that seems to be fighting or combative methods. Having a dozen black belts in karate or some other system seems <you fill in the description here>.

I am not saying those who do have so many grades in so many systems are not experts and proficient its just I can't get my head around how long it takes, today, to get a black belt, i.e. about four to six years, yet I read about some who are not yet in their forties desplaying black belts in a variety of systems, i.e. four to six in some cases, where the math just does not add up.

Yes, once you learn a system really, really well the next one "seemingly" is easier and faster but do those take into account the fundamentals of that system or is it they just add a bunch of forms to their own system and use the current systems name and grade to add to their own system. Seems like a bit of familiarization vs. full and complete knowledge of a system to me. 

I do think that martial artists should enhance their parent system with things that would teach them to counter things like grappling, tuite, or ground fighting, that is part of familiarization, right?

Sometimes I wonder how they decide what to use in a fight or in self-defense since the mind has so much to choose from to counter something in an attack. Isn't the KISS principal more important so when the proverbial stuff hits the fan the choice is there, not the freeze - mostly.

Then again, what do I know. I can barely hold my own in karate. This is just my opinion :-)