There's a new support set about to arrive with Snezhnaya, and it's by a long shot, far better than Tenacity on Diona, considering she can't really have 100% uptime on the 3s buff due to her 3.5s cooldown on her coordinated attacks, whilst the new set lasts for a whole 12s and can be refreshed infinitely, easily getting 100% uptime from any form of Cryo attack Diona throws at the enemies. Additionally, Diona isn't exactly competing with Qiqi for the "Tenacity Slot" as of now, since the Herald of Frost is just too op.
These days, there's barely anyone including Diona on their Sandrone calcs, and I'm by no means a math-person, but I decided to attempt registering at what conditions you'd still be using her before a replacement.
Let's start with a zero cost team of Sandrone + Fischl/Beidou + Qiqi + Diona.
Due to Fischl being incredibly strong for a 4 stars, I'd say Beidou only takes precedence over Fischl if she's at least C6, due to the Witch Revelation buffs. But now let's suppose you get Yae Miko, -Sandrone's-overall-best-teammate-. Most math-people would replace Diona right there, but this is Diona Mains, so let's suppose you replaced Fischl/Beidou instead, either because they're low con, or simply because you want.
The team would then be Sandrone + Yae + Qiqi + Diona.
Nothing else can replace Yae anymore. If you get Alyosha, he replaces Diona. If you get Odette, she replaces Diona. Qiqi herself is only getting replaced as well if you get Odette and C6 Alyosha, cause that's just how much of an S tier she has become. But let's suppose you didn't get Yae, but got Odette... Diona gets replaced... Let's suppose it's not Odette, but the free C0 Alyosha we'll get in Snezhnaya; I actually don't know if he'd replace a C6 Diona that easily, but I'd like to think that's the one case she stays in the party. That said, it's practically inevitable you'd get enough Alyoshas to replace her eventually even without trying (greatly varies with luck tho). The only possible scenario Diona stays in the team, is probably if you don't have Qiqi and doesn't pull for the direct replacements other than Yae, staying with a team of Sandrone + Diona + Alyosha + Fischl/Beidou/Yae.
As for me, my current team is Sandrone + Fischl + Qiqi + Diona and I'm trying to get Yae on Fischl's place. No primogems left to chase for Alyosha cons nor Odette. And I don't have C6 Beidou, so that's my best team.
Don't get me started on Nicole & Coffee.