The thing is, The Bachelor still has three episodes left to air of Nick Viall's season, and Rachel is still a contestant. By announcing Rachel, the show is admitting that she is not the woman to whom Nick gets engaged. ABC spoiling The Bachelor while it's airing is unprecedented, and it's going to make for a weird viewing experience for however much longer Rachel is on the show. This is not say it's going to be bad -- it's still going to be fun -- it'll just be different.
We've never watched The Bachelor knowing who's going to be the next Bachelorette. The Bachelorette is usually announced during the "After the Final Rose" post-finale live show, and often the decision as to whom the lead is going to be isn't even made until shortly before that. One of the most fun parts of watching the show is predicting and arguing about who is going to be the next Bachelorette. ABC has taken away a few more weeks of debate over who would be better: Rachel or Raven Gates (and to a lesser extent, villain Corinne Olympios or likely winner Vanessa Grimaldi). Alas.
Instead, we get to watch Rachel with a new pair of glasses. We'll be watching her to see how she'll handle herself as the Bachelorette. Next week we'll get to meet her family, including her federal judge father, knowing that we'll see them again. We'll watch closely to see where things go wrong with Nick, which will be fun! We love doomed relationships. Just look at La La Land.
Knowing the outcome will make watching Rachel get her heart broken less sad. It'll be cool to see her lowest moment knowing something that she doesn't: She's going to get a huge opportunity for happiness in just a few months.
Plus, it kickstarts a new argument: Should Raven have been chosen instead? People love Raven, and Team Raven folks are already mad that their girl wasn't chosen. (For the record, TVGuide.com is Team Rachel. Raven's nice, but she's too similar to JoJo, last season's Bachelorette. Rachel is a very different Bachelorette, and not just because she's the first black lead in franchise history. At 31, she's the oldest Bachelorette the show has ever had, which may have a more significant impact on the actual gameplay of her season than her race will.)
Spoilers have been a huge part of The Bachelor experience since 2009, when blogger Reality Steve began revealing the outcome of the upcoming season before it even started airing. It's very common for people to watch The Bachelor or The Bachelorette knowing what's going to happen. For them, the destination doesn't matter as much as the journey. Spoilers don't ruin the fun if you think of them as enriching the viewing experience.
Of course, those people are making a choice to be spoiled. In this case, ABC is making everyone watch for the journey. If you're annoyed, that's totally justifiable. But ABC knows you're still going to watch.
The Bachelor airs Mondays at 8/7c on ABC.