I would buy into a new flightsim platform if I had a good reason such as if the current platform didn't do what I needed it to, or it didn't run well on my system. FSX does the job for me and there are plenty of addons so I'm happy with what I have. Some people need things FSX cannot do well enough, or they just can't stand not having the latest and greatest, so they get X-Plane or P3D. Some people just want to have all the platforms. Those are all good reasons if that's what you want and you have the spending power. I think FSX still offers great value for money.
By the way all of the above sim platforms are pretty old technology, they are nowhere close to modern game engine tech and are losing ground. I'm looking forward to the DeadStick sim because it's built on modern gaming tech even though it's for a specific niche audience. If you like bush flying it seems very promising.