Once upon a time it was.... but if you never changed plans I believe you never lost the HD stream unless you canceled and came back.
So many streamers still don't even off 4K at all (Looking at you Hulu) or very limited. HBO Max is awful with that... and things dont stay in 4K. When Tennant was in theaters and on HBO max it was 4K, now it's just in HD.
The majority of people don't own 4K TV's, so I see the logic in branching it out (even though I'm stuck paying more). Since the orange man's administration killed net neutrality, companies have to pay a lot more money to operators for 4K streams so Netflix's cost for delivering 4K is higher. This is why YouTube TV charges an add on for 4K content because their cost to deliver to the end user is higher. Close to double.