As others said, it all depends on your needs and wants.
If you really want a 100-400 "L" go ahead get it. There is no telling how long it will be before a replacement arrives and there is nothing wrong with this lens as it is. The next one will likely cost about twice as much, assuring that a market will remain for the current model, so you won't be risking that much.
Just realize that this is not a small lens. It's not a lens I want to carry around with me all day when traveling. But, if you want to shoot birds or wildlife from a distance, you need this lens.
For all-around travel, I don't believe anything can beat the 70-300 "L." It's the perfect second lens in a two-lens travel/walk around kit (coupled with a 15-85 EF-S for crop or a 24-105 "L" for full frame). It's relatively light and not huge. Generally, I want the extra 100mm of reach that it provides, and I tend to shoot outdoors, so I have taken a pass on the 70-200 zooms.
One additional thing I will say for the 70-300 "L" is that the IS is phenomenal. In a pinch, I've shot it (braced against something) as slow as 1/15 sec with acceptable results.
If I were an event or wedding photographer, or shot in mostly low light, I'd go with the 70-200 mm f2.8, but for me it just doesn't have enough reach. I do have the 200mm f2.8 "L" prime, which is small, relatively light and can give me the extra stops when I need them (if only it had IS).
Like the 100-400 "L," the 70-200 f2.8 is a substantial lens, which can be a consideration if you are carrying it around all day. And, if you are going to routinely couple it with a 2x converter, you're really not gaining anything over the 100-400 "L."
You owned the 55-250, which is in my view one of Canon's greatest bargain lenses. Probably even a better bargain than the 50mm f1.8 and close to the 85mm f1.8 in value for the dollar. It's not the most robust lens, but it is sharp, which is what counts in my book.
If you want something between the 55-250 EF-S and the 70-300 "L," then consider the Tamron 70-300. It's better than the Canon non-Ls. Not quite as good at the 70-300 "L" but certainly not bad and for the money it's very good. The Tamron is about equal to the 55-250 EF-S in sharpness, which is to say it is very good.
I realized as I writing this that I own or have owned too many lenses.