Having observed software and worked in software development for decades, a great sign that a software product is on a final decline is a grand UI redesign. There are exceptions, but it is a fairly reliable indicator. It’s a sign that could mean a number of (usually bad) things: the designers and/or marketers have taken over development, there is no longer internal direction to work on quality or fix bugs, the product people are out of ideas that are feasible so the only thing they can think of to keep it fresh is a paint job.
“A full UI redesign would make this site/software so much better!” - said no end user ever.
You could replace "grand UI redesign" with anything and your statement is still correct. "grand performance update". "grand timeline feature". "grand 3rd party integration". If it's not grounded in an actual user need or problem, then it's a negative indicator. UI design is sometimes a real problem that needs to be addressed, though I'd agree that they only made things worse with Reddit's new update.
“A full UI redesign would make this site/software so much better!” - said no end user ever.