1) First off, it is completely WRONG to say an R5-1600 would not bottleneck a GTX1080Ti. Simply, completely incorrect.
It can bottleneck even a GTX1060 though the amount of bottleneck depends on the game. Some games will have NO CPU bottleneck, and some may lose 20% (compared to an overclocked i7-7700K).
2) Of course in terms of VALUE the R5-1600 might be the best CPU.
3) There is NO SUCH THING as a CPU that is never a bottleneck. What you do is find the best COMBINATION of CPU + GPU depending on your budget. It would be more useful to know the budget for the ENTIRE SYSTEM though.
4) GTA5 is also more CPU intensive than most other games.
5) Intel 8th gen is also nearly here so you may want to wait a bit on that.
6) *You do understand that you will be changing the:
a) CPU
b) motherboard
c) DDR memory (need DDR4 for modern such as 2x8GB 3000MHz DDR4)
d) Windows (usually does not transfer to new motherboard socket)
e) reinstall Windows + programs
SUMMARY:
Since you mention "next year" I recommend you ask this question when you are ready to buy then, as well as give the total BUDGET and exactly what parts it is for.
If I understood correctly you plan to get a GTX1070 so you can "build next year"? I'm confused on that. Why would you put a GTX1070 into a system with an i3-3210 then wait to build the system? You'll lose a lot of performance on the GTX1070 so I would just wait until you have the money for EVERYTHING you need. By then maybe prices will have dropped a bit due to the cryptomining craze (and perhaps an RX-VEGA 56 would be a better deal by then. who knows).
OTHER: CPU usage isn't a very good indicator of whether it's a bottleneck or not. In MOST games an i7-7700K for example might be about 50% even when it's the bottleneck because most games can't fully use all the cores in a good 4-core CPU.
Obviously nearly 100% CPU usage is bad and means the CPU is the bottleneck but that's just not common.
OTHER:
Here's an example of CPU scaling in GTA5:
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/75094-amd-ryzen-5-1600x-1500x-performance-review-14.html
That alone is proof that an R5-1600 can be a bottleneck, otherwise it would be at the top.