Limiting by total income disincentivises companies to spend heavily to produce e.g. films, plays, video games, whatever. At that point, you're effectively saying copyright is only sensible for things produced by a single unaided person.
Sadly, copyright term is the only sensible (and easily enforceable) metric, although whilst I'd agree that the term is currently too long, reducing it to shorter than the creator's lifetime can lead to other weird situations.
For instance, if copyright was just 20 years after something was created, a painting might have been created 20 years ago but never made public until later in their life, and so copyright has expired before they've ever tried to sell it. Maybe there was only public interest in their work later on in their career and suddenly their early work was now considered valuable.
Or if it's 20 years from the date something was made available to the public, then you could reset the clock by creating a slightly different work (e.g. a book where page 200 contained a spelling correction), etc. How would you decide the copyright term then? Would pages 1-199 and 201-end be out of copyright and only page 200 copyrighted?
And assuming we think lifetime of the creator makes sense, then we would need a different rule for companies, because companies can last forever. What we have right now, seems approximately the optimal solution in most cases.
About the only thing I'd change is when nobody knows who the copyright holder is, which is particularly an issue with old software. There really should be some way of legally distributing abandonware if nobody can prove that they own the copyright.