The biggest reason you lose SR for disconnecting is that there are many ways you can disconnect and there’s no way of telling how or why. You could hit alt+F4, unplug your wire, lag out, turn off the PC (forced shutdown or otherwise), ragequit, remain inactive until you’re kicked, and the game wouldn’t know which of these (other than the last one) happened, regardless of your intentions. If someone unplugs their PC because they’re losing and they don’t want to lose SR, that’s not fair to the people staying behind who need to compensate their absence.
Believe me, I’m not gonna defend the devs - they’re sadistic and abusive in many other ways - but this is one thing that truly cannot be monitored by technology as it is right now. If there’s some device in the future that detects whether or not you left by intent, that’s gonna be pretty much the only way to figure it out.
With all of that said, all I can advise you with is to avoid competitive during the hours your internet provider’s systems would be busy. This one is truly not something Blizzard can manage.