I had the exact same thought when I first read about the clear text PIN storage. But, the PIN is only 6 digits. Even if it would be encrypted, with only 1 million possibilities, a brute force attack would be possible anyway.
You are right with that.
Mobile wallets shouln't be regarded as secured wallets.
For me, it doesn't matter whether everything of the wallet is perfectly encrypted on the mobile or the pin is stored in plain text.
I only use mobile wallets for small amounts im fine with losing. As long as your mobile is not rooted its 'relatively' safe (for small amounts).
The only way to prevent this would be using a much longer password, or slow encryption (especially on old phones)
I don't think slower encryption would help at all.
Files can always be moved onto a new PC and be cracked there with multiple graphic cards.
Choosing a 'slow' encryption won't stay 'slow' for a long amount of time, since the technology evolves at a fast rate.