and I agree.
On production sites where uptime is important, it is critical to:
a) Have current, verified backups before upgrading
verifying can be done by copying the production site to a test site, and making sure it works
b) test the upgrade before rollout
although we make every attempt to makesure that the core upgrades work properly, we cannot and do not test every single
addon module to make sure that it is compatible with new versions of the core.... that is the module developers responsibility.
As well, an experienced person with the proper setup and knowledge can upgrade a CMSMS site in minutes once he's confident in the backups. It takes me approximately 10 minutes to
a) ssh into the appropriate server
b) cd to the proper directory
c) view the config.php to find out the mysql database settings
d) run a mysqldump manually to make sure I have a current backup
mysqldump --user=username --password=thepassword thedatabasename > /tmp/mysql_backup_at_the_current_date.sql
e) tar everything up to ensure I have a complete snapshot
rm tmp/cache/* tmp/templates_c/*
tar zcvf /tmp/website_backup_at_the_current_date.tar.gz
f) download the latest file
wget
g) untar that
tar zxvf
h) (if necessary)
go through the install/upgrade routine
i) upgrade any modules that need upgrading
And I have control over each and every step and can see what went wrong, where, easily... and if something went wrong I have the ability to easily restore and start again.
Other developers use different, but similar techniques that they trust due to experience, and for various religious reasons

I don't think they would change them even if there was an 'instant upgrade' button. And since this package is designed by professionals, for professionals, and since an 'instant upgrade' feature would probably cause as many issues as it would solve, I don't see much of a reason to implement it.