Yes, you can set-up a local Legacy server running on a Pi or Windows machine, but as you say that approach won’t buy you an indefinite solution.
Without updates to the local server software it won’t keep-up with Java releases and won’t get security fixes. Because the local server source code is publicly available, it could be updated by people with the relevant skills, but there’s obviously a risk associated with trusting a 3rd party to make these changes.
As far as the phone app is concerned, having it removed from the App Store will be a major issue for iOS users, as most people don’t want to jailbreak their brand new iPhone to enable them to load the app, and it won’t be too long before an iOS update would make the app incompatible anyway.
Life is a little simpler for Android users, as they could side load the app, but newer versions of Android OS and new phone hardware will eventually become incompatible with the Blynk Legacy app.
This will create a situation where users have to maintain an old phone to enable them to still run Blynk Legacy, which negates the usefulness of the app.
There is also quite a large overhead attached to setting-up and maintaining a local server. A Raspberry Pi with an SD card will eventually fail without motive, as SD cards have a limited read-write life. Installing an SSD is a better option, but obviously more costly.
Some skills are also needed to configure port forwarding and DDNS to enable the app to be used outside of the local server.
There’s also work to do in terms of maintenance and backups, and the recent changes to Gmail that no longer allows the use of insecure apps means that emailing Auth tokens, report links etc will no longer work.
Considering all of this, and having run a Blynk local server myself for a while, I wouldn’t recommend anyone going down this route at this stage, unless they need to run Blynk in an environment where no internet or GPRS access is available.
Pete.