First and foremost, it is not always possible to do continuous upgrades, particularly without doing reboots. There are many instances where a new library or module is part of an upgrade, and the module is currently in use with the kernel and some other executables. I have not read anywhere where a kernel option permits dynamically changing from one critical module/library to another.
There are other instances where an update to a program obsoletes a library, because a new library is installed to take it's place. And what about those programs you evaluated, decided not to keep, and un-installedt, leaving behind hidden files or even orphaned libraries.
So, a clean install, keeping /home and /opt and some other critical setting files does wonders for a) cleaning up diskspace, (while we think that ext4 has no fragmentation, it eventually does begin to scatter modules and files around).
b) Dead libraries.
c) New code that could not be installed because older libraries could not be removed due to sharing conflicts.
The con side of my argument is that Fedora is a distribution that supports previous releases for about 1.5 years. -- not a long time. For longer term support, you have several good choices.
a) Centos -- Long term open support of RH linux
b) Scientific Linux see Centos
c) RedHat's Linux
d) Debian and its non-ubuntu derivatives (Linux Mint, etc.)
f) Other distributions.
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