Fresh veggie plate on the side
So Vietnam's climate is typical of the tropics. As such everything is lush green including vegetables and fruits, pretty much year round. There's always an abundance of fresh green vegetable basket when you go eat pho in Saigon and other Southern VN pho shops.
Now that pho is popular around the world, I can see the problem of serving pho this way in many places in the Northern Hemisphere. In many US cities where Asian/Vietnamese communities are lacking, it's just not possible to get fresh Thai basil and culantro (the spiky leaved herb) year round.
So is pho in North America becoming a seasonal dish, where you get some of the full experience only sometime out of the year? I don't know of an ethnic food that is so dependent on ingredients that are so closely tied to the climate of the home country.
I wouldn't go as far to say pho is becoming a seasonal dish. My experience (and by the fact that many pho restaurants are operating year-round in the northern hemisphere) is that people will eat a good bowl of pho even without all the fresh garnish on the side.
I think you're overthinking this. Vietnamese have been serving pho in the US since the mid 1970's, pretty much year round after a restaurant opened. Pho is definite not a seasonal food by any means. Only thing I envy is in VN you get an abundance of vegetables on a huge plate all year long.
We have plenty of history (since late 1970's) to say definitively that pho is not seasonal. It's so good that people continue to eat pho even when not all garnishes (Thai basil, cilantro, etc.) are available. I agree with @limesprouts: you can't get the huge plate of fresh vegetables like they have in South VN.
There's no comparison when it comes to fresh tropical vegetables and herbs you can get in VN. If you're in North America then you'll have to deal with seasonal availability and/or out-of-season high prices. One thing for sure is pho will not become a seasonal dish any time soon.