Although it's hobby thing, i think they still help Bitcoin network if they allow incoming connection (with proper router configuration).
Do they help if the RPi is in a constant bottleneck though? There was a time when portable RPi Zero full nodes were being sold around, and these certainly don't meet the requirements for running a full node and incoming connections might only make things worse.
Running full node client on RPi 0 is ridiculous idea, even though i've seen few people managed to do it.
The lowest possible version of it that could handle full nodes decently are probably from RPi 3 and up; otherwise, it's all just a struggle for both the RPi owner and those connecting to it
Even RPi 3 and 4 (1GB RAM) barely can run one due to memory constraint where you might need to tweak memory usage on
bitcoin.conf.
2GB RAM is minimum requirement to run full node without serious problem.
Now we can't exactly predict size of Bitcoin blockchain but looking at this
Statista chart, I think we are not going to reach even 400GB or 500GB in next few years, and data from
Ycharts is showing 142.6% Average Growth Rate.
But we can calculate maximum Bitcoin blockchain growth with assumption every block full (current biggest block size in KB is at block height 625861 with size 2,422.858 KB) and low average block time (currently 9m40s in last 24 hours).
As you might have noticed already, (residential) internet speed does not grow. It is forever fixated at some arbitrary value like 5mbps or 20mbps depending on how benevolent your provider is (*cough* Comcast *cough*).
...
Which is why I mentioned starlink and Project Kuiper earlier.
--snip--
Unfortunately, not all country have fair customer protection law or prevent ISP monopoly. Project such as starlink will have legal problem on some country.
Next it'll be about data and how much of it is being consumed by Blockchain ledgers, especially Bitcoin.
The data usage will be,
1. Total blockchain size (when you download it for first time)
2. Average blockchain size growth
3. Limitation you apply to upload quota or bandwidth
Ethereum have far bigger blockchain size and blockchain size growth, but almost all people talks about hardware requirements rather than internet data usage.