Hi! Thank you for your active attitude towards the game!
One thing to clarify first is that we never really had weak (or strong for that matter) artificial accounts for the Arena. The reason some relatively weaker accounts appear in the game time after time has to do only with the intrinsics of players distribution on the server. Those accounts never really disappear into nowhere, instead, they progress as well as other players do and join the fray.
The team power calculation is not perhaps the ultimate way to stack opponents against each other, but this is a handy approach to find an approximately similar player to fight against. With an as complex battle system as is now in the game, it is incredibly difficult to produce matchups based on a neat group of factors. But since team power also takes account of Champions' stats, the speed also plays into the calculation.
Please, keep in mind that Arena as a piece of PvP gameplay is supposed to be challenging. You're not supposed just to steadily progress there only through the general development of your account. If a player can't beat their fellows of relatively the same level, defence team power and player power, it means that something should be changed in their offensive or defensive strategies. Unlike PvE areas, where you can get stronger simply by gradually getting better equipment and widening your pool of Champions, in the Arena, you have to deal with a competitive environment of people whose aim is exactly the same as yours and they would not give quarter to anyone.
The point to start with is to analyse your pool of available Champions and approaches your offence team uses. So feel free to share your Champion collection and ask for advice specific to your account. There are a lot of players ready, willing, and able to help you here.