If you get a good system now for say $1000 then that could probably last you 2 years on mostly high settings, then I bet you could make it last for 3-5 years with overclocking. If you get one that just does a little above your minimum now, then it will only last you a few years. So I would say it would be better to get a system around 800-1200 for your needs. I assume you mean medium 35fps at as an average of all your games?
Up to a certain point. Most people don't understand what "top of the line" is. At any point, TOTL will be very expensive. If you were to get a TOTL rig, you're getting a dual-8 core Xeon with multiple K5000 graphics cards. Each of the processors cost $1800, and each of the graphics cards cost $1800. You're basically looking at over $14000 for a TOTL system.
Will it last? Why yes, yes it would. It can run 2160p, which is essentially future proof. Is it worth it to normal people? No. You can buy a car for that price. If you're a millionaire is it worth it? Why yes, yes it is. Buy back to your original question: