A derailleur makes it a lot easier to maintain correct chain tension. Fixing a rear flat by the side of the road is more of a hassle with an internal transmission hub (or a fixie).
I’d argue fixing a flat on a IGH or fixed gear/single speed is easier than a geared bike. Hardest thing about a single speed is needing the proper tool to remove the axle nuts, but after that take the chain off the cog and the wheel will slip right out, just as easy to put back in, just reverse the process. Getting chain tension can be tricky, but tighten the drive side bolt first at the point the chainline has about a centimeter of mesial play and once you tighten the other bolt the chain line should be spot on.
Where a geared bike with a derailleur will need you to put the bike in the smallest gear, and untangle the chain from the cassette. Replacing the wheel back in requires the chain to be properly laced around the cassette and can require dexterity.
Unfortunately derailleurs are necessary to ensure chain tension when the bicycle frame isn’t designed the allow for manual chain tensioning (vertical dropouts).
Also, run tubeless and flats will feel like a thing of the past.
Your description of the way of fixing a flat on a fixed gear sound much more work than the deraileur way. You disengage the handbrake if you have rim-brakes, open the quick-release for the wheel, bend the deraileur outwards a bit and just lift the wheel out. The gear is already at a low gear (does not have to be smallest) since you just got a flat.
Reinstall by moving the casette back inbetween the chain on the same cog and just drop it down and flip the quickrelease back to locked. Connect the brake again if you have rimbrakes. No need to fix chain tension.
I guess it all depends on what you are used to do. I had soooo many flats last year when the spikes started eating through my one year old schwalbe wintertires. 220 spikes makes 220 holes. Had to give up on fixing the holes and change innertube until I could get new tires. Not schwalbe this time.
Don't think tubeless would do any difference here though, see enough people get flats with those too and as someone else stated, the glue can get pretty messy.
Tubeless sucks. I've seen several cyclists on group rides running tubeless tires have flats that couldn't be fixed at all, or required installing a tube to get home. Maybe it works well enough for mountain bikes but for road bikes the tubeless tires seem too weak.
Unfortunately all IGHs are not created equal. Removing a back wheel with a Shimano Nexus with a coast break is a lot harder than removing a back wheel with a Rohloff.
It’s not difficult at all, just release the clickbox, unscrew two axle nuts, unscrew the brake arm. If you have any chain tensioners, slightly unscrew each the same amount.
