The only noticeable time investment is programming the bot.
The bot still has to level. That is a time investment. What happens is, bots are more efficient with time than humans. Bots can do things like level 24/7 and use multiple accounts like you said. However, those things still require a time investment.
If Riot bans early they help the programmers improve the bots, which will increase the reliability of the accounts sold, which will decrease the caution of the buyers.
The same is true if Riot "delays" the ban. No matter what, banning tells the programmer they were caught.
Which is why banning bots in waves is important. It makes the buyer unsure that the account that they bought was undetected or not. If they buy a detected account they're sitting on a ticking time bomb that they can't prevent from being banned.
Banning bots in waves is not important at all. The important part is to conceal detection methods. That is done by introducing a random delay between detection and punishment. Ban waves can actually back fire. It allows cooperation between the bots. "Hey we all got banned at the same time but that one guy didn't. Let's compare code."
Some account buyers assume impermanence. They think, "buy a new account every year". Also, like I said, the longer the bought account remains active, the more valuable the botting service becomes. Riot would have to ban accounts very quickly after being bought in order to discourage account buyers.
But if Riot just bans bots as soon as they spot them, the buyer can be sure that the account that they bought is undetected since it wasn't banned yet.
That is why a random delayed ban is useful. Detection can be immediate. Punishment has to be delayed. The problem is the longer the punishment is delayed, the more harm has been caused. There is a fine balance to be had.
Bot leveled accounts can be supplied endlessly in a F2P game.
Theoretically. In reality, law suites happen. Power and internet bills need to be paid. Programmers have to adapt. The general thought within the security community is "99% of cheaters can be prevented/caught. The remaining 1% can only be slowed down".