I personally don't want my Mac applications looking like that and if Xojo would ever think to implement this, I would probably search for alternatives. There are other cross platform environments around, but all of them fail on the UI due to non native controls, etc.
This is one of biggest plus of Xojo. It looks great on all platforms.
Whilst the concept of native GUI controls for each OS is great in practice we actually get the lowest common denominator because there are so many differences between the three desktop platforms. For example, this is probably why we still don’t have a date picker control without having to spend additional money for a third party solution.
Additionally, because Xojo don’t provide that many controls themselves (because they have to cater for all three platforms with three different GUI toolkits) developers end up with non-native canvas-based hacks to simulate what has already been implemented in a cross-platform toolkit like GTK3.
Personally, I would have no problem at all if Xojo decided to support GTK3 on all three platforms. That would actually speed up development time for me. Much of my debugging time is spent fixing weird GUI glitches between platforms which would probably be solved if there was just one underlying toolkit. I find the logic side of my apps tends to “just work” as advertised by Xojo but the GUI side is often broken.
Additionally, even as a die hard macOS user, I really don’t mind the look of GTK3.
Just my two cents.