A malevolent actor could buy old accounts

I agree each parameter that constitutes a REP could be gamed. A malevolent actor could buy not just an old account but a "good REP" account, after analizing the REP system. In any case it is another level of defense for the social blockchain (compared to plain DPOS).

a benevolent new actor with lots of experience should be able to witness

I don't agree on this point. Here comes into consideration the "community" aspect of social blockchain. That benevolent actor could have lots of experience on other blockchains or even o other community oriented blockchains. But a representative is not the kind of position that is usually of global reach, a witness on a REDPOS would not be just an engineer.

An engineer is a global reach position. It doesn't matter if his previous job was on another company or even if that company was located on another country.

However, a community representative usually isn't "imported" or "exported". If he changes community, he needs to build trust. It's not that a Senator from "country A" then goes to "country B" and can be elected. We don't see that happening on the real world.

That's the reason why a newly created user can´t become witness from day one on a 4 years old chain.

It would be sort of a "conservative flavor" (towards the community) added to the protocol. In my opinion, a valid one.