B'z Longevity

Why I think B'z can stay so popular

(in no particular order)

They write, arrange, and produce all of their music.
They are a live band. They bond with their fans through their lives. They do a live tour every year. Even songs that I normally dislike (Atsuki kodou no hate, Alone, Gekkoh) are SO wonderful live. And then listening to their music later just makes you remember all that emotion you heard in the live. This keeps me going and bonded.
They've been able to adapt over the years to the changing face of Japanese pop. They've evolved from an 80s dance band to a great rock band. I don't know how they did this. After watching both the In the Life tour tape and the Rock 'n California Roll tape, I don't know how the same band could have made both videos. Amazing.
They are able to try many different styles of music and somehow still retain a "B'z-ness" to whatever they do. They have also been able to try different things without alienating their fans. Because of this, they can always present something fresh to keep everyone's interest alive.
Although B'z can and do try many different musical styles, they seem to be able to resist going with the flavor of the month in the Japanese music world. For example, when R&B was really hot, they didn't jump on the R&B bandwagon, they just continued to make the music they wanted to make. This ability to stay true to themselves and not just do whatever happens to be hot at the moment is another reason for their longevity.
They don't stick with one formula just because it works. For example, In the Life and Run were super successful albums, but they didn't keep making albums like that – they tried something different with The 7th Blues. Konya tsuki no mieru oka ni and ultra soul were both very successful singles, but that didn't make B'z feel like their next few releases had to sound the same as those.
Their albums improve with multiple listens, allowing for near endless repeat. And somehow you can always find something new about a song that you missed in earlier listens.
They have an album or song to fit every, and I mean every, mood. It's amazing. Feeling naughty or rascally? Listen to Survive. Feeling depressed and want to stay that way? Listen to Brotherhood. Want a pick me up, songs guaranteed to make you feel better? Listen to In the Life. Pop, dance, pop rock, rock 'n roll, blues, hard rock, jazz, they've done it all! And now that Tak's worked with Misia, I can say he can even do R&B!!!
They pick a style and keep improving it until it's perfect. They kept perfecting their pop rock sound until it ended up with the near perfect album, Run. They then moved on to blues rock, culminating in again the near-perfect album, Loose. After that it was rock, and since the very first album was already a masterpiece (Survive) they experimented with a harder sound on their next two albums. Green was a return to a softer sound, still with a rock edge, but different from anything they've done before. Where will they go next?
B'z have a strong, warm partnership. They seem to have the perfect balance. On stage, Koshi's the dynamic one, the one that gets the most attention, and Tak seems to accept this. Off stage, Tak is the unquestioned leader. Koshi seems to defer to him without question. Since each accepts his role in the group (seemingly without jealousy), they are able to stay together.
Stuff they released 14 years ago (like Bad Communication) still sounds good to this day. Even their first album has a song that I really, really like. With this kind of quality of music, they can continue to gain and keep new fans.
Even after 15 years at the top and 80 million in record sales, they still work very hard at creating good music. They don't seem to take anything for granted. They will rehearse the same amount for 5000 or 80,000 people.
They (especially Koshi since he's the front man) are still able to project a lot of energy and enthusiasm in their music and performances. Look at Makkana Silk (released in 2002) – it has tons of "attitude" and I just love it!
Probably most important of all, they work very hard and are very disciplined. I'm pretty sure that Koshi doesn't have that perfect body by accident. He must take very good care of himself to look so good. He also takes great care of his voice. And I don't think Tak keeps up his guitar skills by osmosis. They both look like they work out and that takes discipline.