I have three active computers that are in use. For this computer, I mostly browse the internet, do some limited online card games, and photo editing. Nothing too CPU intensive. But I also keep my computers around for many years. The 8600 will replace a computer that currently has a Core 2 Quad Q8300 that is barely able to open web pages any more. While I don't need the full power of the 8600 now, I will appreciate the 8600 in the photo editing now and will almost certainly appreciate the speed boost in another ~8 years later when I upgrade again.
I'm just thinking most about value. The 8700 costs about double just to add hyperthreading that in many uses does nothing, certainly nothing worth doubling the price of the CPU. The 7600 was cheaper than 7600K while at a much lower TDP and only 100 MHz turbo speed was lost. I hope the same is true with the 8600 vs the 8600K. My computer is enclosed in a office desk where heat has a hard time getting out and I've noticed that the 95 W Q8300 CPU overheats a lot when doing next to nothing (fans speed up to full speed with just light use and the whole computer desk gets quite hot to the touch). So keeping power down matters. TDP isn't exactly power used, but it is a decent indicator.
It sounds like you have similar needs as I do, and I know what you mean. However...
I know that Passmark isn't a great benchmark, but it does seem a little useful for general back of the napkin type comparisons when talking about surfing and what not. Here's my take on surfing with different CPUs, along with their Passmark scores:
600: Atom 330 <-- Basically unusable
900: Pentium SU4100 <-- Very slow
1550: Core 2 Duo P7550 <-- Feels sluggish, and multimedia heavy websites can be very slow.
2500-2800?: Core M-5Y31 <-- Feels OK but can be sluggish at times
2500: Athlon II X3 435 <-- Feels OK but can be sluggish at times
3000: Core 2 Quad Q8300
3600-3800: Core m3-7Y32 <-- Very good (Note that this CPU performs way better than C2Q Q8300 in Geekbench 4, not too far off the Phenom X6 below.)
5000: Phenom II X6 1055T <-- Very good
5400: Core i7 870 <-- Very good
9000: Core i5-7600 <-- Excellent
12000: Core i5-8400
12000: Core i7-7700K <-- Excellent
It sounds like you are more sensitive to slower speeds than I am, but then again, the i5-8400 has four times the speed as your Core 2 Quad. This is not only reflected in the Passmark score (12000 vs 3000), but also in the Geekbench 4 scores (22500 x 5600).
If you're just talking about relatively light usage, I think the i5-8400 is going to last a good while. It basically equals the 7700K, which was the flagship mainstream CPU last year.