They're a ways from being Delta. One of the things that came out recently regarding crewed launches is Merlin turbines have been developing cracks under test. They're probably only good for a handful of flights.
Musk claims that's all going to be fixed in the next version. We'll see.
That didn't come out recently, that was known from the very start, as Gwynne Shotwell noted in the press conference prior to CRS-10. They have flown with cracks in the turbopumps since Falcon 9 flew for the first time and it was no big deal (customers have known and were okay with it for cargo flights). It's just that for human flights they want every most bit of safety margin they can get, so they'll fix it for Block 5.
To my knowledge there has only been one engine failure (which resulted in the secondary payload being lost), and it is not known whether that was due to the turbopump.
One other thing to consider is that most pre-Block 5 rockets won't fly that often even if/when SES-10 is successful and even lands again. Most customers get factory-new rockets and they already learned a lot on how to make the first stage easier and better to reuse. So once Block 5 comes around the older landed boosters are probably not needed much. They converted one of them into a side booster for Falcon Heavy already, as far as I know, so that may be one point for salvaging them.
It's effectively a version number. SpaceX have been quite inconsistent in how they've named their rocket versions, with Falcon 9 v1.0, 1.1, 1.2, Full Thrust, another upgrade that didn't get a name (informally called Fuller Thrust), and Block 5. Perhaps Elon is just messing with us. Effectively Block 5 is supposed to be the last iteration of the Falcon 9, with a slight performance increase and a lot of improvements to reusability, mostly from what was learned from the landed cores.
I suspect at this point that Elon's entire goal in naming things these days is to troll people. The naming of his autonomous barges after sentient spacecraft from Iain Banks novels is quite clever, and his announcement of 'The Boring Company' should really put to rest any questions of whether he's messing with us with naming things. I find it quite funny, personally.
Also notice the pronunciation Space-sex, and Tesla models s,3,X (he wanted a model E, but settled for 3 after Ford complained). He is clearly trolling.
Even if a booster can only be reused 2-5 times, it's still a dramatic improvement over throwing it away after a single flight and will still have a measurable impact on launch prices. There's also savings to be had by putting all expendable flights on boosters that only have one or two launches left in them.
I think if they can only get two flights out of a launcher, it will not make economic sense to reuse them. But I think SpaceX will do much better than that.
Recovering the engines allows for inspection and modifying the design to compensate for any problems.
Competitors just dump their flight-tested hardware in the ocean, when it is truly a goldmine of test data.
SpaceX is able to do all this testing with a free ride on paying launches. There's no risk to the primary mission, and they get to develop their capability.
There will be failures with reflown hardware for sure. I'm interested to know how well a failed engine can be isolated in flight so not to affect the others.
Musk claims that's all going to be fixed in the next version. We'll see.