If you feel tubeless tyres really are essential, there would be nothing to stop you either sealing the existing rims with the Outex tape system for example, or even getting the hubs re-laced with some more appropriate rims...
Personally for 'all-terrain' adventure riding, this bike with a set of 17/19" rims* sealed with the BARTubless polymer system would be the sweet spot I feel.
*note. There are comparatively few tubeless tyres available in 18/21 inch sizes, but virtually every manufacturer offers tubeless tyres in 17/19 size.
Jx
all has to do with cost at the time of manufacture and that is the time to do this which would keep the cost down any aftermarket fix drives up the cost up to the buyer-it maybe just me but I will not drive any tube tire bike on the street over 50 mph for a very good reason that almost cost me a very bad crash and left me with a very lasting phobia to this day and I don't want to even get into fixing a tube flat way out in the bush or on the side of the road on a 95F day -I just feel in this day and age there is no reason that all spoke rims should not be tubeless