I can't find the study anywhere, but I remember reading an article showing that mice who never digested simple carbohydrates still got cavities if they were injected with glucose solution, causing a glycemic response and leading to type 2 diabetes.

Biomaterial could keep tooth alive after root canal

https://www.theguardian.com/science/2017/jan/09/decline-of-the-dentists-drill-drug-helps-rotten-teeth-regenerate-trial-shows

Dental caries vaccine: are we there yet?

Physiological recalcification of carious dentin.

Having a filling can increase the risk of tooth decay in neighbouring teeth. Risk factors for caries development on tooth surfaces adjacent to newly placed class II composites – a pragmatic, practice based study, Kopperud et al 2015. There is no control group in this study. They could have recorded contact surfaces without fillings to see how well they fair. However, the percentage of healthy teeth developing cavities when in contact with fillings seems accelerated. If everyone had 30% of their teeth develop cavities every 5 years, there would be a high amount of toothless persons.

Until the future is available, you may have to make due with what is available in the present. We would likely have better solutions already, but there is little capital initiative for investing in solutions, much like there is little capital initiative to end wars (think military-industrial complex).

Some cavities are beyond hope and have to be filled in the present, using medieval technology. It is important that they be filled correctly. Composite fillings should be applied in many layers, or they will develop a gap between filling and tooth, leading to the development of a cavity under the filling. Read more

You only need baking soda. You can also mix it with coconut oil and peppermint essential oil, but just plain baking soda works. It's actually not abrasive on the teeth: look up “relative dentin abrasivity” (RDA)1 2. It is a bit abrasive on the gums, however, so having it premixed in oil helps. “Oil Pulling” appears to also be beneficial.

Baking soda is basic on the PH scale, and will neutralize the acidic plaque. The acid created by plaque removes minerals in your tooth's hard, outer enamel, and the PH of baking soda counteracts this. It also acts as an antibacterial, because certain bacteria cannot tolerate the high PH. 12

I'm glad there are some dentists that agree:
https://southaustindentist.com/austin-dentist-explores-the-usefulness-of-baking-soda
https://www.quora.com/Why-do-so-many-people-think-that-baking-soda-is-abrasive-to-the-teeth-when-it-has-a-low-relative-dentin-abrasivity-RDA

I've been trying to find a reason, backed by research, why baking soda might be bad. For example, how high of a PH could be bad? However, maybe the reason I can't find anything, is because baking soda is not harmful.

Commercial toothpaste is bad news. Just look up each ingredient on its own. There are some nicer alternatives without SLS, but I prefer to keep it simple and use baking soda. I use Bob Red Mill's brand because they advertise that theirs isn't contaminated with chemicals. Flouride is bad for you (see www.flouridealert.org). If you have some other toothpaste you use that doesn't suck, get in touch.

Penetrating the plaque biofilm: impact of essential oil mouthwash, Ouhayoun, J.P. 2003

Bismuth subsalicylate nanoparticles with anaerobic antibacterial activity for dental applications, Vega jiménez et al 2017