AVR(or PIC) vs ARM for LoRaWAN nodes
If you product is working, then what is the problem? AVRs are very competent chips, with decent peripheral sets, low power consumption and generally good longevity. The main area they fall down against ARM is in raw processing power, but as you note, this isn't that important in your application. If it is working for you then I wouldn't bother changing it. 

However, one point you are not correct about is cost. There are many manufacturers of ARM chips, and typically not much between them, so the market is very competitive. For example, you can get an STM32F030 for around 50c. It has a lot more power than your ATmega328p and if you need more processing power/peripherals/memory there is an entire drop in upgrade tree that you can swap in. The ATmega328p is not comparable to an STM32F411. The latter has 8x the flash memory and something like 10x the processing power. That it only costs 4x as much is actually quite a bargain.

As to why your competitors use ARM chips? More than likely because there is no reason to not use them and they have become industry standard in recent years. Processing power is cheap these days, and embedded programmers who can carefully optimise code are expensive. Especially if they are a startup, not being too constrained on resources is a good way to accelerate development.