I am mainly looking to prevent the timescale corp. from coasting off of their long past work. In such a scenario, the several years out of date code would not be that much different from the current version, because income for timescale would not have been spent on meaningful improvements.
Another benefit is as a check to the timescale corp. in case they start acting up. In such a case, a large contributor or user might pick up maintenance of an old version and start porting it to newer postgresql versions as leverage. Users of TimescaleDB could be reassured that timescale will not abuse them through the licensing situation because there is some backup plan.