I just read this thread and it's very unfortunate. I used coinomi wallet although i don't store large funds there but i have never been hacked even on other wallets that is "closed-source" i used and currently using.
So in this case the problem i only see is the users' device, or the way or where the backup seed is saved.
The problem is most probably with the user's carelessness and then blaming the Coinomi wallet not build upto the standard.
No matter the wallet is closed source or open source, we need to know the basis on how to keep our wallet safe and unfortunately most people in the crypto world don't know the basics.
A properly airgapped computer has no internet access or contact with anything else that does. Everything you do remains in that local (much harder to penetrate) environment.
You are correct, but to properly set up an arigapped wallet in a completely safe enviroment is hard for newbies to do, and they can easily mess up and lose their funds, hardware wallets are easier for them to use correctly. I have even seen someone suggesting using an airgapped computer that is only connected to the internet once in a while
, once connected to the internet it really isn't airgapped anymore, so on that note i think there is more work to be done generating and using an airgapped wallet than a hardware wallet.
For newbies, even a hardware wallet may not be suitable unless they know how to use it. The best bet for the Newbies is to use a centralized exchange and / or centralized wallet which is controlled by the password and 2fa. Newbies know this stuff and until they learn about the seed phrases, wallets, and stuff the best option would be the centralized wallets.