It is for a lot of people. I know many people who refuse to get a 3/Y because they don't want everything on a single center screen. Also, the rear infotainment screen is an important feature for many people with kids.
I'm not saying that it shouldn't be among the items you consider when making a decision but acting like it's the ONLY deciding factor is kind of foolish when the two vehicles are so different at more important aspects. It's a lot like someone asking you to help them decide between two totally different houses in different areas and your answer is "well that one has a round front door knob and the other one has a lever front door knob" as your only input.
For me, personally, the single screen that looks like the designers forgot to include an interface and realized it at 4:55pm on a Friday and slapped a Tablet in the middle of the dash with some duct tape is pretty damn close to a deal breaker. That doesn't mean if they made a Model S Plaid + with all of the same specs but with this same layout as the Model 3 and sold it for $35k I wouldn't take it and learn to live with it. Fortunately for me, that's not likely a decision I'll ever have to make.
The point is, there are FAR more important aspects to deciding between an Model S and a Model Y. They're both EVs made by the same manufacturer but, aside from that, they're two totally different vehicles that offer totally different driving/ownership experiences. It's important for the buyer to understand ALL of these differences and decide for themselves what weight each of those has for them and their needs.