I would NOT trade in an L 100-400mm for a Sigma 150-500mmm no matter how good the Sigma is on paper. I believe the 100-400 to be better built and have better optics. And this from ZERO personal experience :-)
If you're looking for more reach, you will be saddened when you mostly magnify air and heat shimmer and turbulence.
What you may want is to fill your frame AT THE SAME distance with smaller aircraft. That I would support. But simply more FL for the same of capturing from further away is not very rewarding in our hobby.
If all you care about is capturing and collecting and NOT quality, then by all means go for it. But keep in mind the fact that your new 500mm will seem poor when the images you get are looking very blurry or similar because of the compression.
On another level, it is sometimes nice to be able to fill the frame when a corp or biz jet comes by. But again, your distance is still small.
Just trying to make your life miserable

as I am sure you were almost certain you wanted to buy a new longer tele. LOL
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