For the sanction to really bite, would need to block not just air travel, but people. Otherwise diplomats from Belarus could just fly to e.g. Russia first, and from there to wherever they were going.

There's a train between Belarus and Kaliningrad going thru Lithuania.

Detain every single passenger for rest of the year.

At some point, it doesn't matter.

Qatar was doing massive detours a couple of years ago to keep flying (of course money wasn't an issue for them)

But detouring Bielorussia might be even cheaper than forced landings, who knows.

Who would want to fly over Belarus now?

It wasn't practical for MH17 to fly around the Donbass either but it would have prevented a terrible tragedy.

And there flights weren't even officially targeted. This flight was intercepted by an armed military jet. From there it can quickly escalate in the event of a misunderstanding.

For the specific flight in question that would've been around 60km or 5 minute detour.