After i did a dist-upgrade for my Statler partition, i found 457 upgradable packages in Synaptic. Fortunately we have something for that in Statler's Synaptic. Imagine marking them all for upgrade one at a time...
You can update all available package updates at once with Update Manager (mintUpdate).
The "Mark all Packages.." button was removed from Synaptic because Linux Mint is endorsing Update Manager instead, which selectively chooses upgrades based on a level of safety. The logic is it protects the end-user from any accidental upgrades that may not be safe, and that more advanced users can use the terminal for a full upgrade should they wish to do so.
could you like endorse it in an opt-in way, rather than opt-out?
good on you that you have your own software centre, but keep your hands out of synaptic please? i, and probably many others, really like it. if you guys continue to follow path of ubuntu, which is to subjugate others to own will no matter what, you will lose users in the end, just as ubuntu is losing them.