Going somewhat off topic - sorry.
TL:DR Ethernet speeds above 1Gbps are pointless for Roon and most other audio (and video) streaming applications used in isolation but can make a great deal of difference to the utility of your local network for other purposes.
2.5Gbps ethernet is in no way required for Roon. At least with nothing else utilising the network at the same time. As you say, many network audio endpoint devices have ports limited to 100Mbps anyway and more than this is not required for anything but the most exotic bit rates. Indeed, even ridiculously over sampled 8 channel PCM audio at 1536Ks/s (not directly supported by Roon to my knowledge) and 32bit samples only requires just slightly over 400Mbps. If audio distribution is the only function required of your network you would not need to go any faster than 1Gbps.
However, depending upon the way that you use your network, it can make a great deal of difference to other applications.
Since my two main computers, my Roon Server machine and my NAS all have 2.5Gbps or faster ethernet, for me, it does make a great deal of difference to other network operations. For example, I also use my NAS for storing photographs (Typically 45 to 60MByte RAW files) and video (8k 10bpp). Uploading those files to my NAS takes significantly less time with the faster ethernet being limited only by the read speed (about 2.2Gbps) of the CF express flash card from my camera. Also, with the 1Gbps network that I had before, to edit these files I had to download them onto the local computer and then upload the edited result afterwards. With 2.5Gbps, I can edit them directly from the NAS and still get useable, if not blinding, performance. And, as with the original file upload to the NAS, if I do want to copy a set of video files or a set of raw files from a day out with my camera (which could easily amount to 500GByte or more) to my computer for working on when away from home, then that also takes much less time. I regularly get local network file transfers at 265MByte per second so that 500GByte transfer may only take about 2000 seconds (or 33 minutes) to complete. That is a lot better than the 1 hour 23 minutes that it would take using a 1Gbps network.
To get the benefit of 2.5Gbps ethernet on the local network (for high network utilisation activities not directly related to Roon or other audio streaming) it is not necessary to upgrade your gateway router. Simply upgrade your ābackboneā switch and make sure all relevant devices are connected directly to that switch (including any separate WiFi 6 or 7 access points that you may have if they have a 2.5Gbps or better ethernet port (my Zyxcel NWA210 does).
With respect to Roon, the only advantage of my 2.5/10Gbps network is that there is sufficient capacity for me to use Roon to distribute audio anywhere in the house at the same time as I am performing these other network intensive activities without any performance degradation.
My gateway router does have 2.5Gbps ethernet ports (but only 2 - 1 LAN, 1 WAN) but that is not really necessary since my internet link is limited to 1Gbps down and 100Mbps up and the ONT to which it is connected on the WAN side only has a 1Gbps port. Apart from Internet traffic, the only other traffic that my router sees is broadcast and some multicast traffic on the local network which is not really significant and would not cause an issue if the router LAN port was limited to 1Gbps. Note: I do use a layer 2 managed switch with IGMP snooping enabled and an active IGMP querier so no irrelevant multicast traffic is seen by the router since it is filtered out by the switch. The same would not happen with an unmanaged switch although that would still be unlikely to cause an issue within the context of most home networks.
