Well if everything you're saying is true, This isn't supposed to happen.
If you've truly never been in bronze, and I'm assuming you weren't silver 5 at the end of last season (more on that in a minute), the reset should have kept you above bronze and the 7-3 would leave you above bronze. The only possibility would be if you were around silver 4-5 after the reset and lost your first or second match at this level so you were lowered to bronze 1 mmr and then lost another couple staying around bronze 1 (basically the game gained confidence that you were bronze 1 skill because you lost at silver 5 and went 2-2 or 3-2 at bronze 1 and the last 3-4 wins in a row didn't help because it already figured out you were bronze 1. I guess Alternatively you were low silver but kept getting matched with bronze 1 mmr and thus your mmr reflected that (I really hope you weren't duo queued).
Back to if you were silver 5 last season - you don't get demoted unless you fall a whole division in mmr below your league. So you'd have to hit bronze 5 mmr to fall from silver 5 to bronze 1. You could have been silver 5 with bronze 4 mmr and the reset plus placements brought you from effectively bronze 4 to bronze 1. Heck, if you were gold 5 you could have been close to demotion at silver 5 mmr and lost your early placements as i described above.
As an fyi, getting plat and diamond players in your normals means nothing about your ranked mmr. The two are entirely separate.
Anyways, hopefully you get some good luck for 6 or so Games and pull in to silver quickly. Considering you just went 7-3 just keep up what you're doing and you'll move up soon enough.
Wrt your comment on kda, wards placed, etcc... It just wouldn't work to affect your rank. Players are already too selfish in a team game and it causes losses that could be wins. If your kda counted, those games where I start 3-0-2 as a jungler and total kills are 5-8 I'd have incentive to just afk because the enemy team is stronger and my kda is bound to get a lot worse if I keep playing and I'm probably going to lose anyways, even though I could theoretically carry still and have a chance at winning. Or supports would never die for their carries. Tanks wouldn't peel. People would worry more about their kda than about winning Games. I can't begin to count the number of times I've gone in 1v5 expecting to die but knowing I had to stop them escaping because they were low from Baron/Dragon and once my team got there they'd clean up 3+ kills and it'd be worth. If my kda mattered I might just choose to push a turret instead and let them finish the objective and recall.
