On the one hand.. it kind of sucks to pay to unlock extra capacity.
On the other hand.. the only thing you really lose is ideal capacity, assuming Tesla software-limits it to 70 kWh and not a percentage of actual capacity. You gain increased charge / discharge performance, reduce diminished capacity due to cold / aging, etc.
It also provides a way for Tesla to recoup some additional pricing margin. The extra 5 kWH @ $190/kWh costs Tesla less than a thousand bucks, and if the take rate is relatively high than the cost of transitioning all packs is covered.
Someone who buys the car today may want the lower price; and at least there's a choice for more price-sensitive users, where you can either pay the extra $3k and get 5 kWh more or you can stick with the original cost of the S70 but with the extra perks mentioned above. There's no similar option (today) for 90 kWh packs.
The S75 you buy today is in many ways a
vastly superior car to the S85 Tesla sold a few years ago, and even with the unlock cost is significantly less expensive.