Demos are required as routine to prevent cheaters from getting runs on the leaderboard, everyone has to deal with it to have a fair competition.
Also, resubmitting an already rejected run won't change anything, you are just wasting your time (that you could be using to set up your game) and the time of the mods who have to reject it (again).
Back when the system you are proposing was in place, a person was knowingly cheating and it took the SpeedrunningPolice to come in and fix it. Since then there has been a good system in place that actually makes it easier to do offline runs.
Why is it the rules? Hmm, maybe because people could cheat otherwise? And "drives runners away"? What? It really isn't that difficult to set stuff up, and I'm pretty sure you're alone in thinking that. If you don't want to run with this ruleset, don't. But don't expect the whole community to just accept whatever you do as "legit" just because you can't be bothered to conform to agreed rule sets.
How does having multiple PBs justify making only the top 3 runners submit their demos? It's your own responsibility for not recording demos when the rules clearly state that's what's required; I don't see why you should be excluded from those rules because you lack knowledge of the engine or "the things to go fast" (as if we're just supposed to take your word for it anyway). I also don't see why fluctuation in time of submissions should exclude you either.
Setting up the game and then actually speedrunning it takes considerably more time than creating a few keybinds to record demos, so given you had the time to do all that I find it pretty hard to believe that you couldn't take a few extra minutes to read the guide and write a few lines in the console.