There still were Tablet PCs with capacitive display before though, for example the ThinkPad X60 Tablet had an optional MultiTouch display and it came out in 2006.

Both SL and R-series have been discontinued for more than two generations already. The current ThinkPad series aimed at small businesses are the ThinkPad Edge (E-series) and L-series, the second one of them isn't supposedly that bad. You are also forgetting the W-series, which is the most expensive and which came from the T-series performance models aimed as an portable workstation. Besides that, R-series weren't bad at all, the latest generations were pretty much the same as T-series.