Putting Obi-Wan as the focus of the PT means an abrupt shift by ep4. Because ANH is Luke's journey and when we find out about the connection to Vader it kinda makes me wonder why we would be looking at Obi-Wan so much in the PT since the real story is turning out to be the destiny of the Skywalker family.
The OT was a story about a set of heroes and Luke played the central role. Vader only mattered as a Skywalker because of his late-revealed connection to Luke. Lucas retconned the series as the story of Anakin and by extension Luke. It doesn't need to be seen this way...
No doubt Anakin is an important character (because of his relation to Luke) but that doesn't mean he needs to be the hero of the PT. As BlueCardinal says, up until at least the end of ESB, we don't view it as a Skywalker story. With Obi Wan as hero, we see him early in ANH, he passes the torch, and it works really well as a story-telling device.
Nothing wrong with wanting to tell it as a Skywalker story, but it doesn't need to be done that way. There is just nothing abrupt in moving from an Obi Wan centered-tale in the PT, to him meeting the son of his old friend/student in the OT.
Actually it has to be seen that way because lucas made vader lukes father. That twist introduced in Empire makes the OT a skywalker saga in fact it makes as much a story of anakin's redemption as it does lukes hero journey. So to tell a prequel that focuses on someone other than anakin is jarring.
Its like telling the story of hitler's regime (the original shown) then going back to show how he became hitler, nazi leader but focusing on his high school buddy in art class Johann, who got accepted to the art school that rejected hitler himself (the prequel). The next story (the OS) is about the rise of the 3rd reich so why are we looking at johann in the prequel as opposed to adolf himself?
If what youre saying is the case then the sequel trilogy could be about Lando Calrissian's son's journey while the skywalker children are in the background. You think people wouldn't question that?
I do get where you're coming from, but the idea would be really difficult. Having Anakin as the focus would mean trying to go into his thoughts and progression as we seen it for Luke. That wouldn't be bad if we weren't trying to hide any spoilers. For Luke, we saw him deal with the temptations of the Dark Side and fight for the Rebel Alliance. With Anakin as the main character, we need to see the opposite of Luke. We need to see his progression to the Dark Side and the choices he makes to become Darth Vader.
Obi-Wan as the focus would allow us to see Anakin's fall without revealing anything. Anakin would still be a major character and we could still see hints of the Dark Side in Anakin. It would be more like seeing the fall of Camelot and King Arthur through the eyes of Merlin (which was actually done in the BBC "Merlin" series).
If there is a way to show Anakin's fall and not spoil anything, then I'd use it. Until then, Obi-Wan is the best option for a situation like this.