I really like the later versions of Outlook, you might want to give that a anther chance.
If thats not going to work for whatever reason, consider webmail. If you control your own mailserver you might as wel control your own webmail
. Several really nice programs out there, Roundcube, Horde, Squirrel, but there's others too. Nice thing is you can run them all side by side and each of you folks can choose whichever one he likes. I use Roundcube mostly when I need access to my mail when I'm not at home.
Then there's always the option to keep using Thunderbird. It's not like the program is going to dissapear from your pc's.
But whatever you do, don't give in to Google. Don't ... do it. Don't. Really.