Delta is probably the only travel company I am a "loyalist" for, but OP makes several good points

1) D1 service, both in terms of transcon and internationally, has not been restored to pre-Covid levels, is inconsistent, and is probably behind AA at this point.

2) The 763, while a comfortable plane up front, is not modern and won't be fit for purpose for long. They are changing these, however.

3) The SkyClub situation, IMO, has less to do with offering premium lounges and more to do with simple capacity. The current SkyClub model is high quality enough, IMO, to serve D1 customers. However, in some airports, like JFK, capacity needs to be doubled. My understanding is that Delta is more than doubling JFK T4 SkyClub capacity - first, with a new regular SkyClub to replace T2, and then with a new Delta One club. The Delta One club will no doubt be nicer, but the far more important aspect will be simply expanding space so that lines and crowding are gone.