Seeds at the French and the UO from 1970 to 1979:
FO: Court, King, Wade, Heldman, Melville, Durr, Niessen, Casals
UO: Court, Casals, Richey, Durr, Wade, Melville, Krantzche, Tegart
FO: Court, Wade, Goolagong, Gunter, Niessen, Durr, Heldman, Morozova
UO: King, Casals, Wade, Melville, Durr, Tegart, Richey, Heldman
FO: Goolagong, Gunter, King, Casals, Durr, Wade, Niessen, Tuero
UO: King, Goolagong, Evert, Casals, Court, Gunter, Durr, Wade, Melville
FO: Court, Evert, Wade, Goolagong, Gunter, Durr, Niessen
UO: King, Court, Evert, Goolagong, Melville, Casals, Wade, Morozova
FO: Evert, Wade, Niessen, Navratilova, Sawamatsu
UO: King, Evert, Morozova, Melville, Goolagong, Casals, Wade, Hunt
FO: Evert, Navratilova, Heldman, Giscafré
UO: Evert, Wade, Navratilova, Goolagong, Court, Morozova, Durr, Heldman
FO: No seeds indicated, but I see in the draw (not in order): Niessen, Tomanova, Nagelsen, Jausovec, Turnbull, Bueno, Ruzici, Barker
UO: Evert, Goolagong, Navratilova, Wade, Richey, Casals, Richey, Melville, Morozova
FO: Jausovec, May, Niessen, Marsikova, Tomanova, Hunt, Newberry, Richey
UO: Evert, Navratilova, Wade, Barker, Stove, Casals, King, Fromholtz, Turnbull
FO: Jausovec, Ruzici, Marsikova, Richey, May, Newberry, Ebbinghaus, Du Pont
UO: Navratilova, Evert, Wade, Turnbull, Austin, Fromholtz, Stove, Melville, Shriver
FO: Evert, Wade, Fromholtz, Turnbull, Ruzici, Barker, Marsikova, Stove
UO: Evert, Navratilova, Austin, Wade, Goolagong, Fromholtz, Turnbull, Melville
As you see I prefer to list everything to avoid any biaised view. So we can clearly see that from 1970 to 1973, the French is as decent as a slam as it gets. In 1974, slightly weaker than the UO. From 1975 to 1978, way more weaker than the UO, for reasons that escape my knowledge. In 1979, slightly weaker again, but it gets better.
So, if to you the French is, overall in the seventies, not worthy of a slam, I disagree. I'll say it was pretty weak from 1975 to 1978.
I notice that in those times, you had many grass specialists not attracted to clay and many US players more naturally willing to play the US Open. I will also say that the "weak" period grabbed interest to new players coming on the tour, coming from eastern Europe and favoring clay court game. I certainly find these underrated draws interesting, and they take a nice place in the history of the French Open.
If some players neglected the French then, their loss.