Having a good plan for scaling is absolutely a great move. Changing fundamental architecture later isn't easy. Implementing it all immediately however ..
Sure, I agree, some sort of plan is a good idea. What I've seen many times though is engineers building systems for supporting 100k daily users while the product hasn't even found market fit yet, wasting lots of time on building complicated distributed systems way too early.
Having a good plan for scaling is absolutely a great move. Changing fundamental architecture later isn't easy. Implementing it all immediately however ..