No, "investing" is not the right word, unless this is an actual business for you. That's a dangerous way to think about a hobby. A real investment is a different animal entirely, with an entirely different goal in mind.
Personally I'd look at the 300mm VR and a teleconverter. (drool...) More flexible. IMHO.
Not all investments have to have financial benefits although I understand the point.
I look at all my photographic equipment as an investment in pleasure !
As for lens's. I started with the Nikon 80-400mm VR which I chose in preference to the Sigma because it had VR. My original intention was a pure walk about lens and this lens is exactly that. An investment in a tripod soon persuaded me that was the best way to get decent results in low light conditions but the 80-400 can struggle with auto focus and is noisy too. A foreign holiday in the Gambia made me realise the lens was capable of really sharp photo's and quicker focus when the sun was out. I decided my next move would be the 300f2.8 VR. This lens would be well suited for a walk about as well as being able to deal with the poorer light we get in the UK. The added bonus is the ability to take TC's. I have the 1.4 and 1.7 TC's.
It's a great lens, lightening fast AF without the TC's, very fast with the 1.4 but not quite as good with a 1.7. Am I satisfied with this combination ? No !
The 500mm reach with the 1.7 is still insufficient and the image quality does suffer so I really would like, and I'm seriously contemplating, a 500 or 600mm next.
Have a made a mistake ? Certainly not, the 300mm f2.8 VR is a keeper for life. Very flexible, portable and produces superb images. Ultimately it won't satisfy your desire for more reach though but it will far outperform the Sigma 50-500 in every way ( and should at the price).
Would I trade in the 300 for a 500 or 600mm. Again, certainly not ! The 300 is brilliant for BIF, taking on longer hikes, grab shots from the car, taking as hand luggage on aircraft etc, etc. There will be times when you want more reach though.
Interestingly two friends who have 500 and 600mm lenses have both seen the benefits of my smaller lens and since purchased one.
Me, I just enviously eye up theirs and dream about owning one. Whether or not my desire will be satisfied then I don't know. When you have the biggest there is no where else to go but will it still be long enough.
My other thought is that this is an hobby. I have all the time in the world to spend on it as I have retired. Sooner or later, the opportunity will arise when that target species is close to and a big lens unnecessary. My field craft is improving. Instead of going walk about I now have the patience to sit and wait. It's the least expensive option ! After all , when you have taken all these shots, what do you do with them ? File them away, just as you do your holiday snaps. The real pleasure is achieving the shot and maybe there is more pleasure the longer it takes to get there ?