Agreed. For LOT it takes all of Star Alliance, big ethnic population, and big local market to make CHI and NYC work, and there's really no place else for them to go in the US, except, maybe, LAX, or you might have gotten a PHL if it had stayed in Star Alliance.
https://en.wikipedia.org/wiki/List_of_U.S._cities_with_large_Polish-American_populations
These other recent routes seem to be happening because of connecting opportunities to Asia and Latin America, and BOS may have maxed out for a while.
I was somewhat surprised when I looked up LOT's American destinations that they aren't even at Dulles. They are at Toronto though(another Star hub).
I think the bigger issue is going to be retaining all of the European carriers we have now. I'm not concerned so much with the major players - BA, Lufthansa, Air France, Aer Lingus, Virgin, KLM(dba Delta) but rather Alitalia and Iberia. Even though they do a lot of business in the Boston-Israel market which may go to Turkish, I think Swiss will be fine.
What are the PPD numbers for South American markets? Sao Paulo, Rio, and Buenos Aires are the biggest markets, but there's very little hub potential there.
I'm not going to dig up every South American market but here are the ones you asked about, plus a surprise contender:
Sao Paulo - 51 PDEW
Belo Horizonte - 31
Rio - 23
Buenos Aires - 22
Between Miami, Kennedy, Houston and now Panama City, South America is appropriately covered.
Also to clarify, the number I used for Warsaw was passengers per day, both ways. PDEW is 19.xyz