503 Boeing 737-700 (362 ordered new directly from Boeing)
207 Boeing 737-800 (202 ordered new directly from Boeing)
34 Boeing 737-8MAX
All new deliveries to WN have been 737-800s and Max 8s since December 2011. (
And how long would it take to get 600+ A220-300s?
First, you are talking about at least 15 years to phase out all of the B737-700s because as you pointed out the youngest are only 8 years old.
Second, Southwest is flying nearly half of the active B737-700s and they retired 195 B737-300s with 137 seat capacity, 25 B737-500s and 62 B737-200s with 122 seat capacity, so the smaller jets have been part of their business plan for over a half a century,
Third, it is highly unlikely that Southwest will want to grow the numbers of their smaller capacity jets. On the other hand it is also unlikely
that they will want to take the 143 seat fleet to zero and have an entire fleet of 175 seat capacity. Theoretically, if WN were to purchase the A220-300 the order would likely to be some fraction of 500 jets.
Last June 2019 Southwest averaged 52 round trip international flights per day, and only 9 were using the B737-800. The rest were conducted with the B7373-700. So I don't think it is obvious at all that the smaller size jet won't fit into Southwest's long term future business plans.