Generally speaking, if you're upgrading, you should upgrade to the newest generation because you generally won't get enough of a discount to make the generations between Coffee Lake and the current one a good deal.
While an 8700K will likely hold back a 3070 Ti at the very top-end of performance, it's not going to be so much that it's urgent. The 8700K/9700K/9900K were the best gaming CPUs for years until the Zen 3 CPUs came out and ran 2080 Tis without issues. The 2080 Ti was in the same neighborhood as the 3060 Ti. It's not like you're running a Core2Quad or Nehalem in there!
You could get the 3070 Ti, use it just fine, and then evaluate how much of an upgrade you feel you need. If you're at 1080p or something, the resolution may be more of a limiting factor than the CPU is.