I think a lot of people here are forgetting that there are many people who've been wanting a Tesla, but can't afford one. Not only can't they afford a $70,000, they can't afford a $50,000 car. They can however afford a $35,000 car. So there is a market for the base spec car.
Heck, if you think about it, people are buying CPO Model Ss now in the price range of $40K to $50K. Is the Model 3 going to be so "good" that they are willing to shell out for a up spec. Model 3 when they could get a decent CPO S?
Within the very large car market, the answer is yes, there will be people that want the bigger car and will get a used Model S. There will also be people that want a physically smaller car and they want all the doodads (they don't want a Model S at any price, and will effectively spend anything on a 3 sized car). The point is there are a lot of people with a lot of different needs in the market.
And as we get more options in the market (more pricing levels, more sizes, more body styles, etc..), then the currently distorted market can start normalizing. What I mean by currently distorted is that many of us define a full function car that happens be BEV as 200+ miles of usable range with a reliable high speed charging network. But car buying uses many more criteria than size of fuel tank and prevalence of fueling stations outside of BEVs. Today, using the above definition of a full function car, your option is effectively "large luxury sedan". Over the next few months, the options will be expanded to include "large luxury crossover / SUV" (with towing!).
With a car market that has 2 choices, people are figuring out how to make what they want, work with what's available. There aren't 10+ flavors of large luxury sedan to choose from - there's 1. There aren't any flavors of small-medium luxury sedan - but if you aren't constrained by garage size or something else (ignoring price), then large luxury sedan can do most everything a small to medium luxury sedan can do (anything except fit into small spaces really). And it can do most everything a small-large economy sedan can do (albeit at a much higher price).
And heck - some people that really want a truck can figure out how to get stuff done using the large luxury sedan; that's not usually a cross shopped choice, but when you start with long range BEV and reliable charging network, today that means the consumer has to adjust to what's available. And it's an easier choice using Model X (put the truck bed stuff into a trailer, and voila - crossover and truck in 1!)
You rightly dwell on the expense - there are lots of people that would cheerfully warp what they need and figure out how to get it done using a large luxury sedan. But they can't afford it. There is a gargantuan market of people ready to spend $35-45k for a car, not because the size is such a big deal to them, but because the price is a big deal to them.
There is also a market of people that want a smaller car than Model S, and are willing to pay much more than $50k to get all the bells and whistles. It's smaller than the $35-40k market for sure, but it's also big enough to keep Tesla busy building lots of cars for awhile.
My expectation is that Tesla will hit that $35k mark with Model 3. But if you want to buy a $35k Model 3, then you should mentally plan on waiting 1-2 years after launch of the vehicle for actual availability at that price.
You can probably shorten your time to access at that price by getting your reservation in as soon as they start accepting them.
And - as has been pointed out elsewhere in the thread, the prices on used Model S' are coming down. My claim for more than a year is that the first full function BEV for $35-40k won't be Model 3, and I now claim that it won't be a Bolt or anything else. The first full function BEV (200+ mile range with reliable fast charging network) priced at $35k-40k is or will be used Model S's. Availability of the first full function BEV at the $35k price point has apparently begun happening (I'm surprised at that - I would have expected another year or 2).
Get it now!