Model 3 is already the largest of the "affordable" EVs by interior space. What EV would you choose instead? You mention the Model X. What other passenger EV is as large as a Model X in terms of interior space, or even close, at
any price point?
And Tesla does have plans for an affordable, larger EV: its called the Model Y. You've probably heard of it
It's their top priority new EV right now. Just about to be unveiled, with lots of money getting poured into tooling right after that.
Arguably eliminating the 75D versions contributes to this, and not just simply by improving Tesla's margins. Now they have the lines down at night. Which means at the very least that they could work on line modifications to help improve their efficiency, but even more interestingly, they could now try to merge the old (inefficient) S body line onto the X line. Which would free up the S line. Which could then potentially be retooled for the Y, if Tesla wants to have it at Fremont.
That's in no way saying that this "will" happen. But it is a possibility now that S/X production is reduced and there's some line downtime - a possibility which didn't previously exist.
(Other things that eliminating the 75Ds does: it also frees up more spare part capacity to improve service, and frees up more paint shop and stamping capacity for the Model 3. It also removes Tesla's shortest-range high-end vehicles, which has the effect of making all of the "competition"'s ranges look pathetic)