Not true at all. I have been driving Tesla's sine 2013. Our 4th one is coming by end of this month and it's an S75D. Would an S100D be nice, sure, but is it necessary or even worth $20K, not even close for us. The S75D actually has a higher rated mileage than P85D which I drove the P85D coast to coast and back not long ago without any issues.
What battery you need depends purely on your needs. There is no hard and fast rule to buy the biggest. It could be a total waste of money for some people's driving patterns. You should always look at the driving needs and evaluate both the size of the battery and charging capabilities based on that. For example, say you drive 180 miles in the morning, then come home for few hours, then drive 180 miles in the afternoon. A 100KWh battery doesn't do the trick to live through the day without charging (especially in the winter). A high powered charger on the other hand does the trick for an S75D. Prior to the P85D I drove an S60. In the 2 years of ownership, the battery size was only an issue (requiring to change my plans) twice, but guess what, I would have needed an S130 to avoid changing plans - I remember thinking "this sucks a bit, but even if I had the largest battery size it would not help me AT ALL - today by the way even the 60 would have been fine as there are more superchargers around).
Bottom line; evaluate your driving needs before deciding on the battery size. Of course remember that the Model X is the "gus gazzler" equivalent in the EV world, so for example for us an S75D does just fine even in the winter for us, however X75D would not have worked as well (the 180+180 miles daily driving pattern I mentioned earlier is something my wife does occasionally). If your driving needs including towing, make sure you consider that in your energy consumption calculations too.