Should increase significantly with the release of CoD.

That's impressive. Its increasing very quickly. I could see it overtaking Xbox Live gold subscriptions in the next 2 years.

It makes me think how many subscribers it would have if it was mandatory for online play since its beggining.

We could be talking about 20M subscribers by now.

I haven't played a PSN+ required multiplayer game since buying PS4 and signing up for PSN+ back in Feb. I got PSN+ for the monthly "free" games on PS3 and PS4 and the value in that alone is immense and worth every [insert your currency] you pay.  Maybe if someone only owns a PS4 the "free" games being made available aren't worth it yet. But for someone with at least 2 out of the 3 PS3/PSV/PS4 one would have to question their sanity if they don't have PSN+.

If we assume PS3 subscribers are pretty flat that probably puts the PS4 subscribers at over 6 million, which is almost double the highest selling PS4 game to date (Destiny).

Sure, I agree with the value. I've had ps+ for a couple of years. I just preferred it when it wasn't holding back basic features and splitting the userbase in the process.

Yeah that's certainly possible, maybe even more. Xbox Live has 17m right now, but at the 360's peak, I'm sure it was much more. Sony really made a huge mistake by not charging for online from the beginning. It could have really helped offset some of the huge losses from the PS3. 

I only own a PS4, and fund out that my $50 was a waste of money, as I only also play single-player games.

Those "free" games were not a good incentive for me to renew my subscription, as they were all uninteresting for me, and I certainly would never buy a single one of those if they were offered to purchase.

So, I don't see it as getting 12 games for $50, but wasting $50 on no games... Until they start offering good, interesting games, they are not seeing my business...

This is more or less the way I see it, I've had an internet friend or two telling me of it;s "tremendous", "godlike" value that can't be matched anywhere else yet all I'm seeing is games I don't want and even then they vanish once I stop paying for the sub fee so really they aren't free but locked behind a paywall I'd need to keep buying, even then people will throw the excuse "it's just an incentive", locking something behind and having to constantly pay to use it is not an incentive, just like SvennoJ said "Buying multiple consoles is fun imo. Needing multiple subscriptions is not." and I'm loving the Wii U and PC having no sub and since I'm going with an XB1 I'd maybe be willing to pay their online fee sub but only if it's a game friend own and play frequently otherwise I don't see the point.

That's a 64% attachment.
Decent sure, but not really breaking any ground.

A drop in the bucket to the stupidly fast growth that Steam is seeing. (25 million active users in under 9 months. - Active is defined as someone who has purchased a game and logged in within the last 30 days.)
To put that into perspective, Steam is growing faster than the combined hardware sales of the Xbox One and Playstation 4.
It probably helps that Steam is free and offers free games though, could this be the generation of the PC Gaming Master Race?

I think Sony knew they were losing an oportunity to at least reduce their PS3 losses.   The thing is, I don't think they wanted to launch a $599 console + online fee in the market back in 2006.