I don't think you should be able to be filled as jungler or support currently. Maybe they should start there and see how everything turns out. Would our wait times really be that unreasonable?
The other roles have certain nuances, but for the most part, you're doing the same thing. I used to play a mix of top/mid and occasionally ADC, and it wasn't too hard to switch it up. Sometimes champions I played overlapped depending on the matchup. I've always been useless in the jungle though, and it took a while for me to learn everything about support.
I don't mind getting an ADC that is filled either provided they are trying, which means sticking to the meta. I usually get them through laning phase just fine and I'm sure any support main could do so.
Personally, I'm a support main, so I never have to worry about auto-fill. Since I've became a support main in ranked I think I've gotten mid (my secondary choice) once 2 seasons ago? Maybe twice. However, I frequently see people who are filled as jungler, but maybe that's just my personal experience.
I have so many ADCs that add me after matches that want to duo though, and sometimes it's matches that I just do okay in, but they tell me horror stories about fill supports.
Mage supports are really good right now, but a lot of people crash hard during the laning phase. The match spirals out of control as a result. ADCs rely on supports a lot throughout the match, and while you aren't conjoined twins, you do need to coordinate most of the time.
The reason mage supports are good is because more damage is never a bad thing in this meta, and often times it makes assassins think twice about going all in on your ADC. That's why you should be picking a mage, not because you want to play like you do in your solo lane. I think they get so cocky because queuing up as mid/support means you're going to always get support, so a lot of people just don't do that or become support mains.