There are too many variables to consider to give a straightforward answer.
Here are a few things to consider.
Every year PC Game developers are pushing the boundaries on quality, resolution, realism, textures, physics, etc.
There are some games that go further than others, your typical AAA titles usually team up with either Nvidia or AMD to develop their games to work with those GPUs/CPUs.
So and AMD RX480 with a 2018 game developed with AMD would most likley run smoother than one developed for with Nvidia.
You will find that over the years you will have to turn the settings/resolution down a little bit with each game to keep that FPS at the same level.
Eventually an upgrade is inevitable.
I do understand your question and it is a valid one though:
"What can I do to ensure that my GPU is futureproof for as long as possible with 30FPS".
Within 5 years, a larger percentage of screens will support 4k, QHD or even higher ones like 8k.
At the moment the lowest resolution for 2016 games is 1024x768 (this used to be the max many years ago!)
Resolution will be your biggest factor in maintaining FPS (some may disagree but this is my personal opinion).
If you want to ensure the longevity of a product, go for one that supports the highest resolution possible.
Secondly, ensure you maintain your PC, cooler, GPU regularly (such as formatting HDD every 3 months, clean for dust {not with a hoover/vacuum cleaner!} every month).
And finally, monitor your temps, ensure that you keep them as low as possible when under heavy load.
[edited] also I would add that as games get more developped to use more and more cores, having an i5 is the very minimum recommended to 'futureproof' your PC, the i3 has only 2 cores (albeit 4 threads) so this is really a budget-conscious decision, I presume..?
If you are thinking about gaming on a budget then you would need to dig in deeper into each component and their specifications.
But i5 with a 2016 GPU will keep you covered for probably 3-5years, I'd say by the 5th year you will have dumbed down your resolution/textures/specs to a point where an upgrade would have to be considered.
You could end up 10 years with the same GPU, but then be playing everything on low & with the lowest resolution settings probably being 4k - think...2026, what will we have by then?
Hope this helps with your choice, any further/other questions, don't hesitate!