The form factor is out of fashion, I suspect. It doesn’t fit under all TVs and is unsuitable if the tv is wall-mounted so I doesn’t know how well it was selling recently. Shame though: I love mine. Used it originally as a 3.0 system (I know it has the bass amp/speaker to which really helped the sound) but now run 5.1 which is fine for me. 
I’ve not had Play:3’s but others here have said the One is as good/better. 

Things move on. New features are added. The Playbase and Play:3s were good speakers when they came out and they are still good speakers. 

The formfactor has some advantages too. Before my playbase I had a beam which block the sensor for the IR remote.

And technology, too. The PLAYBASE, as I recall, was an optical input, not HDMI ARC, which meant it would be limited to Dolby Digital, and not work with Dolby Atmos. 
 

I only recently got rid of my PLAYBAR, which was functioning perfectly, and still use a pair of PLAY:3s. They’re still great speakers. 

I have a white Playbase upstairs in my bedroom, with my 43inch TV sat on top. I love it and it's ideal for that room on its own. I have my Beam Gen 2 plus sub and surrounds downstairs with my LG Oled TV. 

Like others have said, it does not really go so well with modern wall mounted TV's plus the optical does limit it a bit. I don't mind that for my bedroom as I don't need Atmos for that room. Only watch Atmos content downstairs anyway. 

I think it was just released too late really hence it's relatively short life. Still a great product though and one of Sonos's most innovative I think 

Spouse has far fewer objections to my One SLs than my old Play 3s appearance.

The Play 3s still sound great but do it from out of sight.

My Playbar is still going strong but I keep looking at the Sony HT-A7000.