Sure, but that's a network cost, not a phone cost, same as maintaining a membership to a golf club.
For many people the cost of remaining in an iPhone-exclusive network may be worth the extra money, for others they don't have any opportunity cost to leaving the iPhone-exclusive network.
Anecdotally, I also thought that my iMessage contacts were worth the extra money, until I found out all the people in the network used whatsapp far more frequently.
Turned out, the important conversations weren't happening on iMessage anyway, which was a surprise to me at the time.