I think you also have to keep in mind the position that de Vries and redbull is in:

• Redbull is looking for a second verstappen-level driver. That's always been the case not only for redbull, but all tier 1 teams: Their aspirations are championships, not points or even podiums.
• De Vries is a 28 year old rookie. That's usually the time that drivers retire or lean on their superior experience to make up for their loss in reaction speed and overall pace. The problem is that De Vries has no experience, while being older than Verstappen by close to three years. The fact that he got to race at all is a miracle: He would have to beat Tsunoda every week by quite a margin to become relevant for RedBull. If he doesn't become relevant for redbull, then why have him at alpha tauri?

Meanwhile they have a young driver in the form of tsunoda which exists in a limbo due to him having nothing to compare against: He could be the fastest driver on the planet in a trash car, or he could be underdelivering without anyone noticing due to the lack of comparison.
This is bad for two reasons:

1. you don't know whether tsunoda is an option for redbull
2. you have no idea how good alpha tauri is over all, which is doubly bad considering that they want to make major changes to how alpha tauri operates.

On the other hand, you have a perfectly good Ricciardo sitting on his hands that performed really well at silverstone. Realistically, you aren't going to lose anything from having Riccardo drive the rest of the season compared to having de Vries drive, but you have to potential upside of more context to the quality of tsunoda and the team, which you wouldn't get otherwise.

In general I'm more suprised that they ever gave De Vries a chance considering his age and the context to his big achievements:
In formula 2 his stiffest competitor was Nicholas Latifi (He won with 266 vs Latifi's 214 points) in what can be described as a dud year after the majority of now F1 mainstays had already graduated (he also needed 3 years to win F2, which is never a good sign).
If you have ever seen an formula E race, you will notice that it is quite a chaotic crash-fest with very weird rules and other nonsense. Just not crashing and not driving to quickly can get you really far by surviving the carbon-fiber mayhems and fuel-conservation issues.
To put it into perspective, here are the race records in the year that De Vries won formula E [1st, 9th, retired, retired, 1st, 16th, retired, 9th, retired, 13th, 18th, 2nd, 2nd, 22nd, 8th] or, in short if we ignore all DNFs we get a mean position of 9th!

In short, there's a reason why Mercedes never even tried to get him an F1 spot: He's not a bad driver, but being "not a bad driver" is insufficient for a top team like mercedes and redbull. There's little incentive to put him into any car, even less so nowadays considering his age.