I think it's a bit disingenuous to suggest finding a 9 yo CPU that still retails at better than 3-400 dollars where in stock, used ones going for not an appreciable amount less used for unknown stress and use for that time frame. Not only that, but the pricing on DDR3 is going far higher than it's worth as well. You can buy DDR4 for less now in many cases. Considering that a $100 i3 10100, a $60 motherboard, and $80 in RAM would perform just the same, or better it just doesn't make sense to discount the time frame in between.
The ONLY way I would purchase a 4790K would be if someone was practically giving it away and I saw it working, personally. It is the only upgrade path for a generation of Intel that was particularly good and still being utilized quite regularly. The demand outweighs the real world performance.
10th gen was the first time I can recall that Intel has actually lowered the resale value on their own product to a desirable price. The i7 makes a lot of sense to purchase now, in relation to traditional pricing schemes.
I don't say this to insinuate that you wouldn't gain performance or (if found at the right price) is a good value proposition, but if you are going to throw that type of money in on an aging system you take quite a chance concerning longevity and relevance.
I am not sure what options your motherboard would allow, but traditionally the Xeon equivalents for the Core i series can be found less expensive as of late simply due to mainstream familiarity with that line. I know that personally I am not comfortable going down that rabbit hole.
I think it's disingenuous to recommend something just because it's newer. Your i3-10100 is going to cost more, require a new motherboard, new ram, downtime to install everything and get the quirks out-and for what--10% more performance that a mild OC on the 4790k will easily match?
https://www.cpubenchmark.net/compare/Intel-i3-10100-vs-Intel-i7-4790K/3717vs2275
Cpus are notably probably the most durable component in a computer, usually lasting well after all other components have failed. The 4790k is no exception.
Considering that a 4790k and 32gb of ram will just bolt in less than an hour (if even that long), the time saved with this upgrade versus swapping the whole cpu, mb, and ram is usually worth more than the cost of the materials to anyone who's busy.
No one is asking YOU to buy anything, so that's great that you won't buy a 4790k--more upgrades for the rest of us.
I'm not focusing on the performance, but the TCO to get that performance. I spend nearly 12hrs end-to-end when I completely upgrade a system, and that type of downtime is not something a lot of people can tolerate. And if they're inexperienced with massive upgrades like this, there's an additional cost of paying someone else to do it and again more down time. The value of the time is not factored in when simply looking at parts.
There is no xeon equivalent to the 4790k--I checked everything before I got one to upgrade one of my Dells. Generally you are right that there are and depending on the bios support using one is trivial, but not in this case.