Wallet developers business model

Hardware wallet developers liken Trezor or Coinkite make money selling a product.

How do developers like blue wallet, electrum or Sparrow make any profit from free wallets? Some wallet developers do it out of believeing in Bitcoin and enjoying the project.Many seem to be a businesses though.

So how do we know the business model for some of these wallets is not to just steal 1 out of 1000 thousand wallets though a backdoor. How else are they making money?