This is very true. Out of NYC, I can always get dirt cheap fares to go to places like London, which is a 6 hour flight. I’m happy to spend $250 - $300 for economy. But, if I’m not intenting to go to London, I use miles. I can literally decide the day of my flight that I want to go next week and change for free if there are award seats. Try doing that on a purchased fare and watch the fees rack up. Or, I can cancel and get my miles back for a nominal fee rather than purchasing a non-refundable ticket. Life has a way of screwing up the best laid plans sometimes, so I’ve done both.
Regarding flying Economy, it’s fine. I used to fly AA to London or Lufthansa to Frankfurt from Dallas regularly before I built up my mileage warchest. If you really want to level up, buy an economy ticket and upgrade with miles to save yourself $5K. I fly a ton but am only Platinum with AA because I don’t fly international for work often, and when I do, it’s to Canada or the Caribbean.