Current industry standard, for those gamers that came in with d20/3.5 and have no prior experience, does tend to be 28mm. Wargame minis might still be 15mm, but most RPG gamers garnered over the last ten years have "grown up" with WOTC minis, HeroClix and similar, which all tend to be 28mm. Those are the guys that probably make up the largest percentage of the customer base, and so I can see why Mongoose would aim their sights on them.
And it happens to work out for me, since I have hundreds of minis at 28mm, and maybe ten or so at 15mm. I'll be better equipped to use my current mini collection if Mongoose goes with 28mm. YMMV, and probably does.
With Regards,
Flynn