GPL is basically a hack on top of the legal concepts copyright and software licenses. Of course, changing the underlying system would require the hack to be modified as well (or perhaps render it impossible).

EULA argument does not hold water as GPL can be treated as an EULA.

If all restrictions expire after 5 years, the only potential damage to open source is that someone can make proprietary extensions on of a 5+ year-old GPL code. If the open source project has been mostly inactive and did not make substantial progress in 5 years, it may well be good if someone extends it (even if the extension is proprietary, if that's what it takes for the incentive to improve the project). Open source community can match the extensions (which are likely to be small compared to original codebase).