I think a lot depends on whether you consider the journey an inconvenience or part of your 40th. If it's an inconvenience then it's a hard sell, beyond "it's my birthday so I get to choose" and "Billy-o can't/won't fly". If it's part of your birthday, then selling points are a visit to Paris, perhaps just for a night/lunch, bar cars (though not brilliant on Eurostar, a bit better on TGV, much better on DB), tables of 2/4 seats for talking, eating, playing cards etc, bring your own food/drinks.
On DB, the beer is on tap, and comes in a glass!
I went from Cologne to Munich a few years ago, and quite a lot South Stuttgart of seemed to be a trundle along at 50-80mph. It will speed up when the Stuttgart 21 project finishes https://en.wikipedia.org/wiki/Stuttgart_21
If you fly, the S-Bahn from Munich airport to the city centre takes at least 40 minutes, and doesn't have toilets.
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