I don't think this new HND Access line idea is necessarily a waste of money, as I completely understand the potential benefit and convenience. But the fact is, it's simply not necessary. While it would be nice to have a Skyliner-type train serving HND and while there's no limit to how convenient we can wish the access to HND to be, the fact is HND is still one of the most well-connected airports in the world. You can take a train right from within the terminal and it whisks you to one of the major city centers (Shinagawa) in under 15min. Even if you wanted to go to Tokyo Station, we're still talking <30min including transfer time at Shinagawa.

HND's current access means cannot be anywhere close to capacity. You can always sit on the monorail or Keikyu coming out of the airport. I most often take Limo Bus from Musashi Kosugi or Tokorozawa. Those buses seat >50, yet they're almost always <10 passengers the entire way. One time I was the lone passenger going from Kosugi to HND.

If JR wants to build this new line, they can. But the government better not chip in. Jpn is sort of the king of white elephants, and they can't afford to continue that trend. Kobe Airport, Shizuoka Airport, various tunnels, even Aqua Line was debatable. I wouldn't file this HND Access train in the white elephant file, but it's still not necessary. All the underground stuff in Tokyo has been ongoing at a frenetic pace, including landfill areas, and one wonders about seismic consequences (computer modeling does not always mirror the actual outcome). And before they start pouring more and more money into HND, somebody really needs to decide on long-term strategy for what they want to do about NRT's role vs HND's role. Right now it looks like they're developing both airports into international gateways in parallel, which to me is rather wonky. About the only other similar example I can think of would be JFK and EWR, although with NYC there is more of a sensible distinction developed in that JFK has become the transit airport for Sky, OW and JetBlue whereas EWR is the transit airport for Star.

You might be right that Shinagawa/Keikyu is at capacity right now and that an extensive construction would have to take place at Shinagawa in order to change that. However, there are already construction plans in place at Shinagawa, at least involving Keikyu. Keikyu will get more platforms. I'm not sure when that's supposed to be completed, but I'm sure it'll be done before 2029.

Kama-Kama project is interesting (funny name). If that comes to fruition, I'm about as good a candidate to use it since I spend a lot of time in Kosugi area and Saitama. Even then, it's really not necessary. Limo Bus is plenty good enough for me as they are. It's ~45min and <2hr from Kosugi and Tokorozawa, respectively, very cheap, comfy, and they take care of your bags.

Again, I would just like to see the following, which should be a far cheaper undertaking:
1). Introduction of Skyliner-type train that will at least get me to Shinagawa (all they need to do is add 1 or 2 cars to each train with reserved seating and luggage space, like the U-Train idea that JR Hokkaido introduced and has been great)
2). Obviate need to take stairs or elevator when transferring to JR at Shinagawa (just install a few more escalators)
3). Have better transfer option from Shingawa heading towards Shinjuku and Ikebukuro (All I want is a limited-stop train like Tokaido, Yokosuka and Ueno-Tokyo lines which all go the other way; currently the only option is NRT Express which only runs qhour from Shinagawa to Ikebukuro, and I refuse to take Yamanote. Ultimately this may require adding another track between Shinagawa and Osaki where the train can then pick up Saikyo track).