I thought the whole reason they did that ending was that you CAN interpret it that they were in purgatory the WHOLE TIME.
Not just the last season in the flash-sideways scenes. If you want to, you can interpret it that they were always in purgatory. Obviously they were in purgatory during the flash sideways scenes as it as-good-as said it, but they left it open to interpretation whether they were the rest of the time.
There is nothing to stop you thinking of it that way.
Edit: At least, that's what the writers said to that american presenter guy
No, the writer's have said over and over again after the show ended that they were absolutely not in purgatory on the island.
Carlton Cuse:
“No, no, no. They were not dead the whole time,” Cuse said, explaining that footage of the plane wreckage at the end of the show was meant to act as a buffer.
“We thought, let’s put those shots [of the plane wreckage] at the end of the show and it will be a little buffer and lull. And when people saw the footage of the plane with no survivors, it exacerbated the problem.
“But the characters definitely survived the plane crash and really were on a very real island. At the very end of the series, though? Yep, they were all dead when they met up in heaven for the final ‘church’ scene.”
We felt the ending really had to be spiritual, and one that talks about destiny. We would have long discourses about the nature of the show, for many years, and we decided it needed to mean something to us and our belief system and the characters and how all of us are here to lift each other up in our lives.
Damon Lindelof:
“For us, one of the ongoing conversations with the audience and there was a very early perception, was that the island was purgatory and we were always out there saying, ‘It’s not purgatory, this is real, we’re not going to Sixth Sense you’. And we felt it too that the show had to become sort of meta in this way.”