Ubuntu – Libprotobuf15 install prompt for removing system packages

18.04aptpackage-management

I have this warning using apt:

$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  python3-protobuf
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

To fix it, I'm trying:

$ sudo apt-get install python3-protobuf
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 python3-protobuf : Depends: libprotobuf15 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

So it asks to install libprotobuf15. I already have libprotobuf10.

When I try to install libprotobuf15, I see that lots of packages that look like system ones are going to be removed:

$ sudo apt-get install libprotobuf15
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  bamfdaemon compiz-core compiz-plugins-default gazebo9-common gir1.2-accounts-1.0 gir1.2-clutter-1.0 gir1.2-cogl-1.0 gir1.2-coglpango-1.0 gir1.2-gdata-0.0
  gir1.2-signon-1.0 gtk3-nocsd hud libavdevice57 libbullet2.87 libccd2 libcolumbus1-common libcolumbus1v5 libdbusmenu-qt5-2 libdecoration0 libframe6 libgeis1
  libglewmx1.13 libgrail6 libgsettings-qt1 libgtk3-nocsd0 libignition-common libignition-fuel-tools1-1 libignition-math4 libmetacity1 libnux-4.0-0 libnux-4.0-common
  libqtpropertybrowser4 libsdformat6 libsignon-glib1 libsimbody3.5v5 libspnav0 libtar0 libunity-core-6.0-9 libunity-misc4 libzeitgeist-1.0-1 metacity-common
  nux-tools python3-blinker python3-feedparser python3-jwt python3-oauthlib sdformat-sdf session-shortcuts unity-asset-pool unity-lens-applications unity-lens-files
  unity-lens-music unity-lens-photos unity-lens-video unity-schemas unity-scope-calculator unity-scope-chromiumbookmarks unity-scope-colourlovers
  unity-scope-devhelp unity-scope-firefoxbookmarks unity-scope-home unity-scope-manpages unity-scope-openclipart unity-scope-texdoc unity-scope-tomboy
  unity-scope-video-remote unity-scope-virtualbox unity-scope-yelp unity-scope-zotero unity-scopes-master-default unity-scopes-runner
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  python3-protobuf
The following packages will be REMOVED:
  compiz compiz-gnome evolution-data-server gazebo9 gazebo9-plugin-base gdm3 gnome-shell gnome-shell-extension-dash-to-panel gnome-shell-extensions gnome-todo
  libcompizconfig0 libebook-1.2-19 libebook-contacts-1.2-2 libedata-book-1.2-25 libgazebo9 libignition-msgs libignition-transport4 libphonenumber7 libprotobuf10
  libprotoc10 ubuntu-desktop ubuntu-session ubuntu-unity-desktop unity unity-session unity-tweak-tool
The following NEW packages will be installed:
  libprotobuf15
The following packages will be upgraded:
  python3-protobuf
1 upgraded, 1 newly installed, 26 to remove and 0 not upgraded.
Need to get 1'019 kB of archives.
After this operation, 73.8 MB disk space will be freed.
Do you want to continue? [Y/n]

Is it safe to proceed, especially regarding the 'ubuntu' and 'unity' packages (I installed 'unity' back just after installing the 18.04 version because I'm on a laptop and I care about vertical space)?
Do you also encountered this?

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