Would putting mattress toppers help reduce sagging?

Hi! So all the mattresses I’ve had have sagged relatively soon (between lots of sleeping, sitting, and not buying high quality mattresses i assume). And I’m looking to buy a new mattress. So, I was thinking about getting a mattress topper to put on the new mattress? Like find a really good one, and when the topper starts to wear down replace it. Or would that just not work like I’m thinking/be too expensive over time??