I'm sure that for those who are difficult to fit, a personal last makes both ecomonic and practical sense.
But I very much doubt that to be the case for someone who finds a really good or even pretty decent fit in the RTW line. Painful break-ins are by far the exception for me within the overall RTW context involving multiple different lasts from different makers in different styles and often different sizes (UK 8.5 to 9).
The classic last in a G width in 8.5 UK fits me quite well right off the rack. I was fitted by Phillip at a trunk show and he suggested 3 small last adjustments which were incorporated into an order for an MTO chukka. The fit on that pair is absolutely spot on perfect. I don't own a better fitting pair of shoes and frankly don't know how shoes could fit any better, be they custom last or even full bespoke.
Again, I'm not disputing that some really need a personal last for a decent fit. And of course, if you are planning several St. C. purchases over time, then the cost of the personalized last amortized over multiple pairs would be fairly neglibible. But if your existing St. C. pairs fit you well (which you would know better than anybody else) then a personal last is likely far from needed.