A good question to ask before starting a startup is : "Do I see myself working 10 years on this problem?". Looks like they ran out of steam, rather than out of runway.
Why 10 years? Is it not typical with startup founders to plan a lucrative exit in a much shorter time frame?
7-10 years is the realistic time frame for a moderately successful exit, usually those that are "Bought" after 1-3 years are actually failed ventures with all or most of the money going back to the investors to recoup their loses.
It seems high at first but take a look at practically any successful startup (Slack, Dropbox, Notion, etc) and they all were around for 8-12 years before the founders cashed out.
> "Good startups usually take 10 years." - Sam Altman
It also aligns with other Y Combinator teachings, such as targeting growing markets.