Nostalgia is all well and good, but it can't be the basis for how anything -- a life, a group, a genre -- is defined. We all see writers (and musicians and artists and sports-people) we loved and admired go out of print and out of fashion. We can mourn that. We don't have a right to try and reverse it and to force our memories and tastes onto everyone else. Nor do we get to declare everything that isn't as we recall it Bad and Wrong. This is how history goes: things change, tastes change, we move on.
It's a point I wish the current UK government would take on board, too.
All lists of 'canon' encode privilege. They are drawn up out of existing privilege. They are in many ways an attempt to reinforce privilege (this is one reason why I am on some level always slightly suspicious of canon!). Like you, I wish more people were still reading Zelazny and LeGuin and Bradbury and Delany, who were the golden writers of my teens and twenties. But I know people now in their mid to late 30s who bewail that the young are not reading Banks and Gentle and Gibson. We all have our own doorways into these other lands. And the more diverse those doorways are, the better. The two books I most want to take back in time for my teenage self are Nnedi Okorafor's Zahrah the Windseeker and Malorie Blackman's Noughts and Crosses because they say things about who people are, what people are, how people are that are critical and necessary and beautiful.
'Young writers' itself... So much coded there, about what is valorised in our culture. The demand for 'youth' itself privileges certain groups over others -- it's much easier to grab the time needed to write and acquire the confidence needed to put the work out there if you're a middle class white man. Women, people of colour, working class people, people with disabilities of all kinds tend to find it harder to get that time and confidence. So they debut older (and find themselves told they're irrelevant, too late or have somehow missed their turn, sometimes, too, which is... Well, it's a thing).
Heinlein mattered, certainly. But he is not the whole of genre, nor its blueprint, and we have grown beyond his visions and words and ideas. He doesn't need to go on mattering. This is history, alive and breathing amongst us, moving and changing shape. And that's how it needs to be.