She had a top 10 hit in late 1996, nearly 35 years into her career at the age of 54 years old. 

Her 2014 album "Partners" went Platinum in the US, Australia, and UK and was the 14th best selling album in the US that year.

The album performed better in the UK than albums released by Katy Perry, Lady Gaga, Taylor, Beyoncé, Miley, Britney Spears, etc. released around the same time.

People will try to malign her as making for music for "old people", but don't take into account that her audience naturally grew with her and most of her fans are Boomers and Xers (while she's a silent) and nobody of her generation has had the same staying power.

Streisand is the only artist to have number one albums in six different decades.

I think only Taylor will eventually surpass her in terms of longevity as a female artist (in the US). 

No everyone considers her a Legend. 

Not really, she's like the example of longevity.

She’s still literally the benchmark for longevity and multimedia cross-over success.

 

She’s not underrated at all.