I think the idea of an extensible copyright is not to allow some people to maintain the monopoly for longer than normal but to have a mechanism for things to enter the public domain earlier if the authors are no longer interested in distributing the works themselves but mainly use the copyright to stifle others.

This way you can set the base copyright duration shorther than you could otherwise without people complaining that some works take a while to become profitable.

That said, I agree that it isn't neccesarily the best idea is it predominantly benefits the already well off who can better afford to gamble on the copyright extension fees.

Personally I think zero copyright is probably the best since society benefits from sharing and remixes from day one. Look at how quickly memes propagate and consider if copyright was actually enforced there. Now consider if the could have the same creative mobility for the kinds of works where copyright is enforced.

But I also don't see zero copyright as attainable within my lifetime as it would upend so many business models. Shorter copyrights (with or without extensions) are much more realistically attainable as for most copyright holders it might change the equation a bit but won't require them to immediately overthink their funding.