Thing is, as I see it, if a new keyboard doesn't make you play completely new stuff, make you play better than you did before, make you do new techniques you never tried before, how is that musical growth?

An average piano player on a Steinway D isn't a hair better than on an old Samick. But a great player on a Steinway D is STILL a great player on an old Samick!

99% of it all, in the end, is still YOU.

To be honest, most modern MOTL arrangers, let alone TOTL arrangers (and even some BOTL arrangers) are capable of wowing anyone in the hands of a good player, and of providing all the sounds needed for just about anything you need for the foreseeable future. So why not just take whatever energy you expend on researching, lusting after and procuring new toys, and channel it into the PLAYING instead?

That's where any improvement is going to be the most apparent anyway. Short term, sure, your audience and you might notice a change. But after a few weeks, it'll wear off, and if you are still playing the same old licks and tunes you played before, the same way, there won't be anywhere NEAR the improvement that all that money spent was SUPPOSED to bring you...

Focus all you energy and attention on PLAYING, not gear, and while you may not be a much happier person for it (wish I could play like Herbie Hancock a LOT more than I wish I had an Audya!), you'll certainly be a better player for it..!