Anecdotally, five strings are still pretty rare in US orchestras. I think five strings with high Cs are becoming more common in solo or crossover situations, but low Bs are still pretty rare. My gut feeling as to why that's still the case is a bit of a chicken/egg problem: luthiers over here don't interact with many five string basses, so building one or doing a conversion is going to take more time (and by extension, money) than adding an extension, plus with a five string there are elements of an instrument's construction that make some basses much better candidates than others. From a player's perspective they're more of an unknown because of the rarity of good five string basses, and an extension requires much less to change about your approach to the instrument since you can easily "turn it off" if you're not using the low notes. I haven't spent a lot of time around five string basses in general, but my feeling as to why high Cs are more common among players is that while there are the same setup/construction limitations as a low B (tighter string spacing being the main one), the lower four strings aren't further away from the player like the top strings are on one with a low B, so the C is easier to ignore if you don't need it. I'm kinda spitballing though, so I could be very wrong there.
I've probably mentioned it here before, but I do think five strings are a more elegant solution than an extension for an orchestral situation because of how classical and romantic composers would often double the cello line verbatim in the bass part, and with an extension there are many more technical hurdles than there would be if I just had a low B I could pop down to and continue playing as normal (see: Shostakovich 5 movement 1 at #22). Alas, that is not the reality in the US, and I really like my Laborie and am not in the market for an orchestra bass.
Although I don't play many jazz gigs anymore, I do/did appreciate having the extension for playing low pedal tones or when playing hits, but when playing basslines I pretty much never used it. I personally felt having a five string BG was way more important in jazz/fusion settings that required BG than in situations where a DB was preferable; the extension was fun and nice to have, but it was definitely not essential.