Yes, exactly, it needs I availability and, very important, no P, U or T fare (or any award ticket fare, like X), even if that's a "comfort flex" category.
I upgraded a ticket with miles just 1,5 months ago (well, that's when the flight was, the upgrade was requested a while before that). The only odd thing is, that you have to call in to get this upgrade (I find this especially strange, given the fact that you can request an upgrade with miles on a Star Alliance partner airline by filling out a form online, as well as given the fact, that these use the same inventory as award tickets and this could have easily been done through "manage my booking").Here's my take on how good a deal this is: At first glance it does seem like a good deal. However, since you have to have booked at minimum an S fare, this already costs around 90 Euros or 100$ for a 1000+ miles international route (not including a discount or taxes). If award availability is open, you can book this for 15x the distance, which means anything from 15.000 to usually 19.000 miles. The 10k miles for the upgrade you mentioned correspond to issuing a 18750 miles award ticket from the beginning (since it's 8x the distance for the upgrade, 15x for a full award ticket).
So if by buying that 90 Euro ticket (or a bit less if you get it discounted) you can use another 10k miles to upgrade, then you basically saved 8k or 9k miles by paying 90 Euros, which in my book isn't worth it and it's also a worse ratio than the "10k miles for 100$" you mentioned (unless of course you don't have the miles for the whole award ticket to begin with, which is a different story).
The main problems are two (and they are often present at the same time):
- ​​​You need to find that S fare to qualify for a miles upgrade and not pay too much. But award space is usually non-existent, when the cheapest fare is S or above. Also, if S isn't available, but award space is, then there's definitely no debate about which option is best (just go for the award ticket directly).
- The other issue is, where I availability is open, most of the time the available economy fare is a U, or even a P. There is a way to get the S fare to appear (booking via phone is not recommended, since you pay 20 Euros on top, basically going back to the point where this is simply not worth it), but it's tricky and in all honesty, having successfully done that once, I would rather not waste that much time and get my nerves cracked again (due to anxiety of perhaps not making the booking, before someone else reserves that award space).
The ticket I upgraded with miles, that I mentioned (if I remember correctly this was the very first ticket I upgraded with miles in my 5 years of constantly redeeming miles for tickets!), was an S fare which was already discounted by I think 20% due to a general promotion and I used a 30 Euros coupon code to get the net ticket price down to exactly 35 Euros (plus 32,69 Euros taxes ex-Athens, but you pay these anyway if you book an award ticket too). So instead of redeeming 18250 or so miles for the award ticket, I paid 35 Euros and redeemed just 9968 miles to get in a business class for which I also had a paid ticket (this time with a 30% code) which cost 135,80 Euros plus the same taxes. See the difference here? 9968 miles for precisely 100,80 Euros (well ok, the fully paid ticket also got me about 1700 more miles!).

I hope this didn't get confusing, the bottom line for me is: It is rarely really worth it, especially if you value a redemption for award space at that price (which is the same value I'd also give it). You're almost always better off booking an award ticket in business class straight away.