Can you please provide references to all of your numbers? a P90D for only $120/mo more doesn't seem quite accurate I also haven't seen too many inventory P90Ds with a discount of 25k. Sure, if you don't want a new car (aka 5-15k miles on it) then maybe but then you aren't really comparing the same thing. Additionally, the Model 3, fully loaded, with full range, is very unlikely to be less than 50k. So, it is very likely the S60 (larger car, more luxurious) will still retain it's value well. I think the only thing being canabalized will be their own new sales of the S60 (which they will probably discontinue and replace with the 3).
So to me it seems, the S60 is a big model 3, 2-3 years before you can get a model 3, for 10-15k more than a similarly apportioned model 3 would cost you 'today.' The big difference, of course, is you are talking about waiting to buy a car which you don't know the features of, you don't know what the interior (or even exterior?) will look like, and getting it in 2-3yrs (hopefully?) to buying an existing car, you will get within 60 days, built brand new, that is bigger, for 10-15k more.
Truthfully, the S60 doesn't seem like a bad deal at all!
If I couldn't afford an S60, then I wouldn't buy it. But if someone can genuinely afford an S60 they should. It is a great deal.
ALSO, don't forget if you put your model 3 deposit in now you will almost certainly not get the $7500 tax rebate. So, the S60 and possible model 3 are actually much closer in price.