My MXT goes as deep as my V3i, both in the air and in the ground. I've done a fair bit of comparison between the two, including swapping out the exact same coil on each machine. VLF detector technology pretty much topped out 20 years or so ago and there haven't been any big breakthroughs, just adding bells and whistles. The V3i is loaded down with them, which makes it a fun machine to tinker around with, but at the end of the day, the MXT performs as well and gets as many keepers as the V3i. This is coming from a guy who owns both and with lots of time on both detectors.
So IMHO on the original question, all the big brands have stagnated for the most part, and I don't think White's is really outclassed, they just haven't done any updates or come out with new machines (except the MX Sport which is a rehashed MXT) lately. Give someone a mid-upper end machine from any of the big manufacturers and once they learn that machine's quirks, they'll be doing about as well as with any decent machine.
If there's not going to be any big breakthrough for awhile, which is probably a safe bet, the manufacturers ought to really focus on tweaking their existing designs. White's ought to spend some time really squeezing the most out of the V3i which I think is an awesome detector, just needs a little more polishing of the firmware to make it more responsive, especially in the Disc audio. If they did a really well-thought out update of the V3i, they could ride that another 5 or so years while they quietly work away on the next big thing.