Onepage checkout, or not Onepage checkout

I've implemented a multi page checkout on Magento previously and to be honest it proved to be more hassle than it was worth. I suppose it depends a lot on how much functionality the alternative checkout needs to modify (and if it uses any of the existing onepage checkout code). Personally I'd always be in favour of keeping the checkout as vanilla as possible as it keeps upgrades and maintaining modifications to a minimum. Checkout is arguably the most important part of the site so you need to make sure it is as robust as possible and for me sticking with the default onepage checkout is the preferred way of doing that.

It became kind of standard to implement OneStepCheckout over default Magento's One Page checkout.

I don't think OneStepCheckout is better. In some tests I did, default One Page checkout outperforms OneStep in terms of conversion rate, in some cases it's the other way around.

There has been several extensive usability researches in 2012 on these checkouts and it seems that number of steps itself is not a problematic part. You can read more about these studies here.

The only major problem of Magento's one page checkout is that users perceive it as separate pages, thus sometimes hitting the back button to go to the previous step, which takes them to the cart instead of the previous step. You can see this behavior if you're tracking the funnel through Analytics, a lot of users seem to return to cart from different steps of the one page checkout. If this issue is fixed (enable back button functionality during one page checkout) I'm sure you'll outperform any OnePageCheckout extension out there.

First of all I really hope this question is not intended to promote neither 2 extensions you mentioned, nor cheaper alternatives. If so it has to be shut down.

The fact that "everyone" is replacing OPC is a bit exaggerated.

The reason one may do it is the fact that simplified layout of the checkout page may possibly increase the conversion rate. However, this fact is questionable and might not be worthy spending $500+ (there are cheaper OSC extensions available but only 2 extensions you mentioned in your question are reliable).