There's no requirement, just your use case. Your 3600 is perfectly suited for what I assume is your use case (games) and I'd personally hold onto it until the next full sets of hardware come around (AM5 or real 10nm Intel, DDR5, maybe PCIe 5 though that's probably ways off on personal computing).
If you do stuff that likes more cores, then you have a reason to get more cores, where exactly that ends up on the i3/5/7 etc scale is just marketing.
Of course not, let’s face it, there are plenty of people in London that have super cars... Generally it’s a matter of balancing what you need, trying to add some extra to allow for future-proofing, and what you can afford.
Unless you're running something 100% 24/7 I don't think it needs to be a consideration, i3/5/7/9 etc aren't designed for different/specific tasks other than in the fact ones with more cores will perform better at tasks than can utilise more cores etc. Nothing wrong with having a 24 core cpu and only run things that use a single core, it's up to the user in the end. Until Christmas the 4 most powerful cpus I owned were ones that spent 99% of their time unused not even in a board, same with gpus. I've decided to use some more now, but still my most powerful cpu is sat in its clamshell on a desk just chilling. I never felt the need to upgrade so didn't bother.
Ryzen 5 & 7 are the same architecture - Ryzen 7 will have two CCXs and Ryzen 5 either has 1 and a half or two CCXs each with a core fused off ( I can't remember). The efficiency is more or less the same - IPC difference will be negligible.
I like to look at it this way. You spend more to be on the cutting edge (eg. i7/i9), then if you are okay with performance, you won't need to upgrade as often to keep up. For example, I have i7 2700k. When overclocked, it can be quicker than i5 7500. Meaning I didn't need to upgrade to play HL Alyx. Whereas if I had an i5 2400, its multicore performance may not be able to work with latest multi-core games.
'Tiers', or 'i5/i7/ryzen5/ryzen7' are pure marketing ****. Ignore it entirely, as it tells you naff-all about actual performance. Instead, decide what you're going to be using it for (gaming? photoshop? 3DCG rendering?) and look for benchmarks in those applications to see which one actually does the job you need it to do best for your budget.
Or what your wants are. I want, for instannce, 5GHz and a LGA socket on my next build, so it's unlikely to be a Ryzen chip.
Its a preference I have Personally I've had to bend back pins on a PGA CPU, but never on a LGA motherboard And the socket retention is VASTLY better!
