I don't know if this has been mentioned yet, but it seems really important:
Boston can no longer get anywhere close to max cap room next year. The best they can do, if they waive and stretch Horford, is about $25M in cap room. And that's not enough for Beal unless there is a Jalen Brown trade incorporated.
It's worth noting that Boston generated a $17.8M TPE in the Fournier transaction, so it looks like the Celtics are actually going to go ahead and operate above the luxury tax using the TPE, MLE and such to add to their existing roster.
With Boston off the table as a plausible destination, I'm much more confident about resigning Beal, even if he doesn't agree to an extension in October. I don't think he'll want to go play for Detroit, OKC or Charlotte while missing out on the 5th year that we can offer.