I politely disagree with your interpretation of D&D 5e RAW.
Nowhere that I know of states that domains are locked to specific gods.
There certainly are suggestions regarding domains, but the final call is up to the player and their ability to justify their choices with the DM.
Additionally, as stated in the thread above, the Life domain is singled out as being available no matter what god you worship.
Personally, I enjoy the idea of mixing and matching domains and belief systems in ways that I don't feel that I have been encouraged to think of them before.
For example, the goddess Shar is associated with the domains; Trickery and Death. However, after reading her long and storied history, it could be argues that a Cleric who wields the power of; War, Twilight, Arcana, Life, Light, or, Tempest, may choose to worship the Dark Lady. Some may even argue that there are Clerics of yet more domains who may worship her. And don't forget that there are those who believe that she and her diametrically opposed sister, Selune, are actually the same goddess!
What I think we can both agree on is that Larian is great at giving us more options than we even expected while still honoring the lore of D&D! <3
