Sustainability should be regarded as the ability of a practice to be continued.
Continued for how long? Without limit, except as posed by events truly beyond the control of those performing the practice.
Is there a limit beyond which it is not possible to "continue"? Assuming this planet as the horizon, one would expect time for many millions of generations. Other events beyond our control may occur that make life here impossible. But why count on it? For a civilization to be sustainable it should be able to continue while the sun provides a suitable environment.
The term "more sustainable" is commonly used even though it is misleading. "More sustainable" suggests a practice that is suitable for a longer period than previous practices. Such a longer period offers more time to implement practices that can indeed be continued. That is worthwhile only as long as it does not divert attention from the effort to come up with a sustainable solution. No matter how much effort is spent on finding "more sustainable" improvements, someone will eventually have to come up with a solution that can be continued.
"More sustainable" improvements so far seem to postpone serious problems by no more than one generation (that is a rough assumption, but I can't easily think of examples that do much better). That is a far cry from the millions of generations that may be possible.
Nature shows that it can be done. Every civilization should try. Most have failed so far. We know more now. Admitting that "more sustainable" is not sustainable may help.