Don't worry about viablity of single bolt guns - they work perfectly well. In facvt, not much looks flimsier than the single top-bolted W.R gun but it works - it was even used in double rifles without incident.
The Purdey bolt with top-lever operation via a Scott spindle has become the most-favoured method of bolting a sporting shotgun since its introduction in 1865 but it is no faster to operate than a side-lever and no stronger than a snap under-lever (either with Purdey bolts) or a rotary bolt.
The combination of looks, ease and speed of operation, stength and longevity are all in its favour but much of its popularity is conservatism - everyone wanted a gun that 'looks right' and matches what everyone else has. thus was so much with the development of the sporting shotgun.
There are of-course, plenty of top-lever operated guns with other bolting systems. They may not have found as much favour as the Scott/Purdey but most of them work pretty well.
