What sort of shortcomings? I've been using TSW a lot more than TS the past few years, DTG have spent a lot of time fixing bugs since it was originally released
Lack of realistic quality sounds which are a big thing for me in terms of immersion, lack of flexibility in terms of choice of rolling stock. E.g, why can't I use the class 52 and Mk1's on the Paddington-Reading route rather than being limited to crawling around at 25mph with an express engine? Equally the 37's from the Tees Valley on the Trans-pennine? Lack of a steam locomotive to use on the West Somerset Railway, lack of editing tools to make my own scenarios or use others. There's no moving cars either on roads. It's visually stunning, but how long has it been out now and there are still glaring omissions. I like the 3 British routes I currently own, but I'm not prepared to invest further in 2020 (at the prices they're asking) until you can do more with it or they supply realistic sounds (they exist, so why don't they talk to the main supplier of custom sounds, that they already have a working relationship with...).
Meanwhile in TS2020, I can run pretty much whatever I want wherever I want, with accurate sounds and accurate and plentiful AI.