Hi kenagross2,
You can see my thoughts about buying a mattress with the intention of adding a topper in post #2 here. While adding a topper can be a good idea if in spite of “best efforts” you end up choosing a mattress that needs additional softness … it also adds to the risk of a purchase because choosing a topper that is a good match for both you and the mattress you are using it on can be almost as challenging as choosing a new mattress in the first place.
A mattress topper combination can certainly provide the flexibility (similar to a component mattress) that allows you to replace just a single layer instead of replacing the entire mattress but a mattress that is a suitable match for you in terms of PPP would be an even more important part of the “value” of a mattress purchase IMO (see post #13 here).
Phoenix