Now, in just .5B years Earth would likely become uninhabitable due to Sun becoming a red giant. In other words, on Earth life spent 90% of its total available time before sentience emerged. So on one side life is constrained simply by time, and on the other, sentience might not be necessary for organisms to thrive: crocodiles are doing just fine without one for hundreds of millions of years. To think of it, it is only needed for those who can't adapt to the environment without it, so humans really might be very special, indeed.
[0] https://www.quantamagazine.org/intelligence-evolved-at-least...
Humans have been just as smart as you and me, maybe even smarter according to cranial measurements, without inventing anything that significantly changed their way of life.
There could be loads of planets with prehistoric humans, having a fine time hunting with bows and picking fruit.
How could we possibly know this? The only case of "Dolphins" we know of, is on Earth, with the interference of humanity, and we're looking at Dolphins at a really small timescale.
Given N thousands of years without interference from other species, who can really confidently tell exactly how Dolphins would evolve?
(warning, this is one of the most depressive pages on Wikipedia)
[0]: https://en.wikipedia.org/wiki/Timeline_of_the_far_future
[1] https://en.m.wikipedia.org/wiki/Star_lifting [2] https://www.orionsarm.com/eg-article/4a48d58c84350
Even if all industrial activity stopped tomorrow there's now enough CO2 in the system to guarantee a succession of uncomfortable and expensive droughts, floods, storms, and wildfires for thousands of years.
If it doesn't they will become more and more extreme very quickly.
If ocean acidification and warming destroy the foodchain in the seas, collapse on land will happen very quickly.
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC4494450/
If you're referring to technology/civilization-building capabilities, that is a different matter.
The Sun will not become a red giant in 500 million years.
