That is what you believe. My experience is different. We had a time, when a number of different ships were seen as very viable for combat. Both PvE and PvP purpose. Those ships were VASTLY different. Some relied on more speed, bettter suited for boom-and-zoom tactics, others relied on more maneuverability. Some relied very much on their shields, others even fought completely unshielded. The placement and grouping of weapons and even the position of utility slots, when using tools like point defense, were a huge factor on what people prefered.Frontier doesn't know how to do that. Players can't even agree on what makes ships just "different" and not better or worse. The only way that new ships won't make old ships obsolete is if they are worse than the old ships.
That may not be what you are saying, but that is what you are asking for.
And over several years, there was no clear superior choice, which everybody agree that it would have been the apex combat ship. If you want more info in this, ask Morbad, he was way more into the details than anybody else who still is around here, i think.
Reaching such a point, where such a huge variety is viable, definitely is not easy to reach. A number of mistakes have been made over the last years, pushing ED away from this. But it's not like it is impossible to do.
Unless, of course, we all are convinced that it is impossible and demand that FD would not improve things. Which seems to be the idea of some people here.