I wish Jolla can do something about the possibility of disable the device lock so dont need to reinstall for Xperia phones

[rozgwi]

tl;dr: What you’re asking unfortunately is not possible since that’s not how encryption works.

About the lock code

There’s a reason why you’re required to use a lock code when having an encrypted device*:
When installing a security door on your house, it’s of no use if you leave that door unlocked or with the key in the keyhole.
Since your storage gets unlocked when booting the phone, the only time your data would be protected without a display lock code would be when it’s turned off.

* At  least as far as I understand. Hopefully I'm not missing some aspect about this.

About the possibility to ‘disable’ encryption

When researching the encryption technology involved (in case of Sailfish OS, this is LUKS), and setup options there’s mention of a detached header: Simply put it stores the key necessary to decrypt your data on an external device (e.g. USB flash drive). You can read up all about it on this Medium article.

The point in case: Sailfish stores the decryption key for LUKS on device. I don’t know the details (haven’t enabled encryption yet) but supposedly with an encrypted boot partition (cf. ‘Gold Solution’ on the article mentioned above).
What this means: Decrypting on-device is very difficult and extremely volatile to data loss. Take, for example this warning from the related entry on the Arch Wiki

Note: As of 2020, and version 2.3.3, when using cryptsetup to decrypt a LUKS2 block device the program requires you to provide a LUKS --header file. If you do not use the “detached header” feature of LUKS, and naively try to pass the block device itself (which contains a LUKS2 header) as the subject of the --header, cryptsetup will accept this and go ahead with alleged decryption. Afterwards the block device will show up as a LUKS2 device with no key-slots, and YOUR DATA WILL BE LOST. If you try to use cryptsetup luksHeaderBackup as the header file used with --header , YOUR DATA WILL BE LOST. If you try to restore a backed-up header after this faulty decryption, YOUR DATA WILL STILL BE LOST.

=> Assumption: There is no detached header available for the LUKS setup of Sailfish OS. Thus, there is technically no possibility to undo encryption on device. Reflashing is needed.

A possible workaround

There seems to be a work-around to avoid mandatory encryption:

@tomin

There is a way to do this. After flashing you must boot the device to recovery mode, open shell and remove file /var/lib/sailfish-device-encryption/encrypt-home from root partition. With that removed, it won’t create LUKS container for home partition on first boot. I don’t recommend doing this but it is possible to skip the encryption on first boot this way.

On a philosophical note

Security and comfort most of the time are - especially in the context of information technology - mutually exclusive. The guys from Open Whisper Systems have done a fantastic job and some enormous effort to create such a comfortable app as Signal has become.

Maybe it’s good to keep this in mind. Unrealistic expectations with this can quickly lead to frustration - although from a technical perspective everything is working as it should