Gravel bikes are going to transform in coming years as now they are neither very comfortable nor fast. It turns out that wider MTB tires are both faster (when it comes to rolling resistance) and way more comfortable and safer than gravel tires even on tarmac, let alone any kind of terrain.
The reason is that soft casing is way more important than knobs when it comes to rolling resistance and MTB tires can be made soft because all the additional rubber coming with width while gravel tires are usually harder and thus slower (and less grippy and less comfotable). Notable exception is Continental Terra Speed but it's still not as good as Race King and way more puncture prone.
The problem with most current gravel frames is that they don't fit wide and fast MTB tires so you are stuck with the worst of both worlds - slow, not comfortable and still not grippy/soft enough to go fast off-road unless it's a very well maintained road.
Gravel bikes are much more comfortable and fast than road bikes on unpaved roads, which is what they are designed for. Wider wheels will add more weight and more angular momentum, making it harder to climb and harder to steer so you might need to replace the drop bar with the MTB flat bar and without the drops you can move the saddle into a more comfortable position as well... ending with a hardtail w/o the suspension fork.
Kenda booster & rush. Continental race king. Maxxis Aspen & Aspen ST. Schwalbe RRs, Vittoria Mezcal, S-works Renegade & fast track, Pirelli Scorpion. Basically every MTB tire manufacturer makes an XC race tire in the 600g range. Some are below 600g, some are above 600g (but below 700g). Just look at the list on the website this thread is under.
But these do not show amazing rolling resistance on that site either. Only Race King approaches top RR numbers of the gravel tires. There are plenty of sub 400g gravel tires with less RR than these. As far as I understood my correspondent, he predicts that riders will add angular momentum (when and MTB riders already complain about steering on gravel bikes) and, apparently, won't win any rolling resistance. What for? Riding at even lower pressure? To do what?
Salsa cut-throat is a gravel bike, fits 2 inch tires. Personally, I find having drops to be more comfortable because you can change hand position. Look to tour divide bikes for gravel bikes optimized for comfort and fatter tires. I don't understand your prediction for an imminent transformation.
My prediction is that gravel bikes will have wider forks and people will put MTB tires in them instead of 40mm "gravel" tires they do today. It will be faster, more comfortable and safer. Drop bars are great, they will stay. You don't need an MTB bar for your typical light off-road terrain.
Gravel bikes with wide forks and MTB tires already exist. That was my point with the salsa cutty example.
Classic gravel tires do not have very aggressive knobs, but there are some that do.
Gravel is odd as that can be hard packed dirt, small rocks, larger, up to mountain road (babyheads). For some of those conditions, the sidewall of a MTB is overkill and there is a stronger desire for a supplier, faster tire. Basically a cross country MTB tread.
So, there are already wide forked gravel bikes, and there are conditions where a 38 tire is plenty. To fit a 2 inch or 3 inch tire also requires trade offs for the down tube. Which is to say, it's arguably different bikes you want fir some Kansas gravel compared to northwest mountain passes.
As well, the upper limit will probably be at 2.5 inches.
Last, a lot of MTB tires are designed for folks that drive up to a trailhead and then ride down a trail. Those tires need to be solid, and just solid. So, I think it will be the variety of tires that change. More tires suitable for "light" cross-country MTB and ranging in between the spectrums.
I absolutely love my Lauf with their weird suspension front fork. Bike is super comfortable to ride. Also has plenty of width room for wider tires. I run WTB Nano 40c, which are a great mix for road/gravel.
>It turns out that wider MTB tires are both faster (when it comes to rolling resistance) and way more comfortable and safer than gravel tires even on tarmac, let alone any kind of terrain.
...except when you are going fast. But I do agree that honestly, full on XC MTB are better than any gravel bike for mixed terrain. Modern race XC bikes are not only lightweight, but have enough suspension travel front and rear to tackle super rough trails, which means you don't have to rely on tire pressure for compliance (although QC on the carbon frames is questionable even from the bigger brands).
That being said, when it comes to road, ultimately, aero bikes are still king for efficiency if you are least moderately fit.
The reason is that soft casing is way more important than knobs when it comes to rolling resistance and MTB tires can be made soft because all the additional rubber coming with width while gravel tires are usually harder and thus slower (and less grippy and less comfotable). Notable exception is Continental Terra Speed but it's still not as good as Race King and way more puncture prone.
The problem with most current gravel frames is that they don't fit wide and fast MTB tires so you are stuck with the worst of both worlds - slow, not comfortable and still not grippy/soft enough to go fast off-road unless it's a very well maintained road.