What I don't understand ist why it makes any sense to design an NIC or PHY especially für 2.5GbE. When NBASE-T was announced, everyone complained that the difference only lies in cabling and the tech needed was the same as 10GbE, only with lower frequencies due to cabling. Since that, too, was only necessary for distances above 55m, it would only be of benefit as a fallback mode for large buildings with old cabling, while homeusers had high chances of always achieving 10GbE.
When Aquantia presented their AQC107/108-NICs, everyone pointed out that it was the exact same NIC, just that the 108 was limited to PCIe x1 instead of x4 and thus to 5GbE. The Chip itself was identical in size. Aquantias reference layout AQN107/108 showed as much with the layout being nearly identical apart from PCIe. Thus far, only the AQN107 has been adopted by many companies. Since, Aquantia has presented the AQC111C/112C and AQC111U/112U, which are 5GbE and 2.5GbE-only-NICs for PCIe3.0x1 and USB3.0 respectively. The AQC111C has a much smaller footprint then the AQC108, so it seems there may be a benefit in being limited to 5GbE only, but that could just be because of just having one instead of four PCIe-Lanes. Again there is no difference between AQC111 and 112 apart from Speed.
Same with Broadcom, which by the was ony seems to offer Multigig PHYs, not NICs. Their 10GBASE-T/NBASE-T-offerings are of the exact same size as the corresponding 5GbE-PHY with the same amount of Ports.
From this I would assume singleport 5GbE-NICs could have a small advantage in price and usability compared to 10GbE-NICs by using only one instead of four PCIe-Lanes, but apart from that the requirements for chip and board should be the same. I don't see any benefit at all in being limited to 2.5GbE only. You could plug a 5GbE-NIC in every System that has at least a PCIe2.0x1-Port (standard since AMD AM2+ and Intel S1155) or USB3.0 (same or even earlier on Intel S1156), while 10GbE needs at least a PCIe2.0x4-Slot (x2 electrically would suffice) or USB3.1.
So, I can only guess there are some cost benefits I don't know of for Intel and Realtek to choose 2.5GbE for massmarket instead of 5GbE, or, as someone said, they want to milk this cow first and only offer 5GbE later on. But, since with PCIe4.0 one lane will be sufficient für 10GbE, the small price benefit for 5GbE will soon disappear.
But sadly we can't do much about it. If Intel and Realtek really offer their 2.5GbE-NICs so much cheaper than any 5GbE-offering and not much more expensive than current GbE-NICs, we should soon see Routers, Switches and Mainboards that offer only or atleast some 2.5GbE-Ports and don't cost much more than GbE-only-offerings, while faster speeds will remain too expensive for most users. That really is a shame because 2.5GbE will limit even one regular 3.5"-HDD, let alone SSD, while 5GbE wouldn't limit the former and even the later not so much.