There are three different things a higher tech unit can do:
– Fill a new role in the army.
– Add an alternative for a role in the army.
– Replace a role in the army.
As far as RTS design is concerned, some of the best designs are for higher tech units to fill new roles (worthwhile ones) in the army or to add alternatives for existing roles. Those two help to create more strategic and tactical options for players.
However, replacing a role is limiting to strategies available. Note that this isn’t the same as adding an alternative, though an alternative can sometimes be better, and then become a replacement. Preferably, an alternative should be an option that has its unique advantages and disadvantages which are different than those of the other option. Each option should be useful in its own way, not making the others obsolete due to simply outclassing them.
One example is Ion Cannon Frigates vs Attack Bombers in HW1. Both of them are meant to take down large ships, but Ion Cannon Frigates have more damage output and durability while Attack Bombers have more mobility. ICFs are more prone to enemy anti-slow ship weapons, and ABs are more prone to anti-strikecraft weapons. ICFs take up frigate slots and ABs take up fighter slots.
Preferably, both options should be desirable choices for the role, but for different reasons. However, if one outclasses the other to the point of almost always being the superior option, then the other option adds little to no strategy to the game once both options are available.
Now, in Homeworld, class-based ship capacities kind of help to avoid this compared to games that have a total supply capacity for all army units, because you can have one option without taking away the capacity to have the other option. However, you still have to choose what to fill your fighter and frigate capacities with, and what to spend your RUs on.
When designing the multiplayer, let’s try not to replace options with higher tech options. Let’s allow higher tech to enhance the army and to add options without taking other options away.