Huh? There's no such thing as a "737 pilot", there is a "pilot with 737 type rating". It's the whole "becoming a pilot in the first place" part that's hard, not certifying a new type. Plenty of pilots are certified to fly multiple types. The fundamentals of flight are the same across all planes.
The airlines would've just had to retrain their existing pilots to fly the new type. It's not nothing, but it's a lot less worse than massive lay-offs and rehiring.
I think you're severely underestimating the time, effort, and money involved in retraining into a new type rating. Usually the process is that the pilot takes a job running air-freight in their new desired type rating. There isn't any money in that, either for the pilot or the airline. You're correct that the fundamentals are the same, but the regulatory burden is enormous.
If getting a new type rating was cheap, this whole 737 max debacle never would have happened. Boeing would have just told everyone the 737 line was done, and to retrain into 767s or 787s, and for anyone who wanted to stick with the 737 to pound sand.
If it was cheap and/or easy for Boeing to tell airlines and/or pilots to get a new type rating, they gladly would have sold them 767s which do everything the 737 MAX does. (it does) They've probably already told them that and they've said no. And Boeing made the 737 MAX with the same type rating as the regular 737. Which is why we're here.
The airlines would've just had to retrain their existing pilots to fly the new type. It's not nothing, but it's a lot less worse than massive lay-offs and rehiring.