I find #FOSDEM sponsors generally bad but also very easy to ignore at the event. Probably they have little leverage because the budget is so low (thanks to volunteers and ULB).
An exercise in collective action I'd like: a crowdfunding to "buy out" all non-pure-FLOSS sponsors. If the target is reached, FOSDEM VZW cancels those sponsorships and gets the funds from the pot. (The first year you need to pad the offer due to the extra work created. Afterwards you can book early at a lower fee.)