19 inch 350 trials pattern tubed tyres are readily available.. though not by manufacturers you may recognise. 18 inch 400 tubed tyres are now a rare thing and should be snapped up as no high street manufacturer makes em any more.
i d whole heartedly support standard bikes but that bird has flown the nest as too few could agree on what was standard and what would be acceptable. the 18 or 19 inch wheel for example .. your argument may be pragmatic but it also instanly lowers the gearing and thus makes a bike easier to ride in a section.
i wrote an article for the yc newsletter some time ago detailing the mods to the welsh championship winning cub of about 1960.. practically nothing was the facts but no one .. no one would ride such a bike today never mind win a trial on one..
the passage of time may have made the bikes older but thier riders/ owners
have more money and time, resources abound in 2014 literally anything can be had at the end of a phone and so these riders with time and money spend both on thier bikes.. and the wheel turns.