Will come back to you with post and lower-end (webGL1) ‘graceful adaptation of the experience’ but I’m in line with guru;) @PatrickRyan in the post above. This ‘graceful adaptation’ is also a point of importance for me (actually, a must-have). There is no doubt that working with soft shadows (your experience) will have a great visual impact when switching to webGL1. I’m afraid you’ll need to make some compromise or even adapt your lower-level experience.
Meanwhile, I just wanted to quickly share with you a point that can be of importance to get this design/effect you are looking for. You can work the ‘angle’ of your light (casting a shadow) and also adjust the ‘exponent’, bias, distance and normal bias (be careful though with some of these entries, some are very sensitive and need to be adjusted up to 3 decimals). You will get very different results and at some point (close to Math.PI) the angle of your light will switch (as if it had a reflector something). Working the angle and the distance of your light(s) casting shadow(s) can really help getting this smooth and long shadow effect. Again, preferably a WebGL2 targeted solution and as said above, it will be a bit costly on performance. But the effect is good in my opinion.
Here a couple of quick screenshots from my scene:
This sounds like a good compromise for your case.