It is closer to 40 years old, but in this case that doesn't make it irrelevant because disease vectors don't change if that makes sense? It was before there was a rotavirus vaccine available.
They looked at 102 infants who were ill and 2,587 healthy infants to compare. So overall nearly 3000 children which is a large sample size.
And yes exactly, it seems that the protective effect from breastfeeding does tend to postpone any potential infection until babies are older and more able to fight it off - so much better than nothing, but the vaccine does give better protection than this. Remember that if you're looking at how nature builds in its own protections, the only real way for this to actually affect things is natural selection, and unfortunately natural selection doesn't need ALL babies to survive, just more than whatever condition causes them to not survive. So while science isn't always better than nature, in some cases it is because the goal is different.
If you look at the whole world, which includes many developing countries, diarrhoea is one of the leading causes of child mortality. The biggest factor in this is access to clean water. This is much more important than either breastfeeding, healthcare or vaccination. So we are very lucky that we don't need to worry about this in our country. However there still remains a risk and vaccination is one way to reduce that risk. It seems like both breastfeeding and vaccination are helpful and they each help in different ways, so it is worth doing both, which is why it's recommended for all babies.
This is interesting/helpful in explaining how different interventions help reduce the risks of different gastro-enteric infections.
https://ourworldindata.org/childhood-diarrheal-diseases
And this explains why the vaccine was developed and how it helps manage risk even in developed countries with clean water and good sanitation - they say here that without routine vaccination, 95% of children have had at least one rotavirus infection by the age of 5 years old, and around a third of those children will develop a severe case of diahrroea. The UK started to vaccinate against Rotavirus in 2013. Bear in mind that the UK still doesn't vaccinate against chicken pox, suggesting to me that Rotavirus is considered more dangerous than chicken pox. The fact that they adopted this vaccine 5 years after the US, whereas the chicken pox vaccine has been standard in the US since 1995, makes me think it's probably a good idea to have it if you have the option. Remember as well that if your baby can be vaccinated, that helps ensure better protection (herd immunity) for other babies and other people who may not be able to be vaccinated themselves. The more babies who are vaccinated, the lower the case numbers in general across the whole population. This is likely because toddlers are spreading this kind of thing rampantly at nurseries, as it's contagious 2 days before vomiting starts. (Then it runs through families, workplaces etc.)
https://www.cdc.gov/vaccines/pubs/pinkbook/downloads/rota.pdf