(Which is not a spoiler, folks. It's just a fact.)
Here's the thing: Companions leave.
I think that it's always hard when your first companion leaves. Just as hard, in some cases, as losing your first incarnation of the Doctor. But that's the nature of Doctor Who. Companions leave. Some die, some marry, some get fed up of the constant death, some get stranded in alternate dimensions. Some are left behind to start their lives over. But the fact is: companions leave. Change is your constant. And we'll miss them (well, some of them), but we'll love the new ones that come along, too. After all, I started off with Romana and Adric and Tegan and Nyssa, etc. etc. But that doesn't mean I love them MORE than Rose and Jack and Mickey just because they were there first. (In fact, you don't want to get me started on Adric and Nyssa. You really don't.) And yes, your first companion is special because he or she is your first, the same way your first Doctor is special cos he was your first. No-one is saying they weren't special. Or that you're wrong to grieve. But that's not the same as saying no-one else can be as special, in a different way. Who knows? There could be a decade or more of wonderful companions in store for us. On this series, people change. It's just built into the concept.
Now, yes, people who are solely Rose fans and possibly only in it for the Rose & Doctor relationship (however you perceive that to be--be it romantic or filial) may leave. And that's fair. If that's their only draw and the only reason they're emotionally invested in the programme, it's understandable that they'd perhaps not want to stay once the show they fell in love with evolved into a different dynamic with different castmembers. Speaking as someone who adored first season of Angel, but really only vaguely liked the series it later became through a complete cast change and retooling of the premise, I can understand that. It's not that the other Angel series was necessarily a bad series. In some ways, it was better. It was certainly better crafted, and had some excellent actors, and it had a lot going for it. But it wasn't the a hardboiled noir detective show with a quirky sense of humour about a vampire, a demon, and a prom queen I had fallen in love with, had lost my head and my heart to, and mourned when every trace of it vanished completely.
So for some folks, I can see why, if they signed on for The (Possibly Romantic) Adventures of the Doctor and Rose wouldn't feel the same about The Adventures of the Doctor And Other People. But that doesn't mean they might not fall in love all over again with The Adventures of the Doctor And Other People. Let's not discount the possibility, k? Have a tiny bit of faith that the same team of people might actually bring you more characters you love, played by excellent actors, and just as well written, and you might just lose your head and heart over it as much or more than the show you first fell in love with.
Rose and Jack and Mickey and Jackie are just the beginning. Not an ending. And different isn't always bad. It can be just as good. It can even be better. But let's not throw out the baby with the bath water 18 months before we have to.
ETA: susanmgarrett told me at Gally this year that (paraphrasing here) the Doctor is Peter Pan, and Wendy always goes back to the nursery and grows up, and he always comes back and takes her daughter. Because that's who he is. And Wendy lets him, because that's who she is. And it doesn't mean she doesn't ever regret her choices. But it is just the way things are. And I think that's an excellent comparison.
Also, I personally think of the TARDIS as Tink. But that's just me.