If an anime ends before the manga that it was based off is finished, could it hurt the popularity and legacy of the anime?

It depends on how well they excecutes it, and also how well the fans accepts it.

If they adept it faithfully without cutting corners and was well received, the fans will always keep supporting it by either buying the manga/light novel, or the blu-ray/dvds. The best example for this was To aru Majutsu no Index, where the 3rd season of the anime is planned to be aired this year, which almost 10 years after the last one.

On the opposite, if the anime was such a mess, people will surely dropped it and move to the next one, and only loyal fanbase of the series that keeps supporting. Lots of examples of this one, but the latest disastrous one was "My Sister, My Writer". The light novel was OK, but the anime was such a train wreck, it instantly become a meme on the internet.

Anime such as these are basically just over-blown commercials for the manga. They are meant to promote the manga and boost its sales. This has been the norm for a long time now, so an anime doing that now is nothing new and would have very little relevance on the popularity or legacy of the anime. It's how anime work in the first place. In fact, it is quite rare for an anime to actually complete the story.

bleach be popular for a long time and i cant wait for episode 367 if possible. anime producers were too fast at making anime an tite kubo was working his butt off i think. they decided to stop and take a break but after a while the motivation for it was lost.

Highly unlikely. It just leaves room for a sequel.