Hi diamondshopper,
A 30 year warranty is more about marketing than reality because with good quality materials (such as latex) foam softening and the loss of firmness over time and the resulting loss of comfort and/or support is the biggest reason that most people will no longer sleep well on their mattress and will need to replace it … not the loss of thickness or visible impressions which is much less common. There is more about mattress warranties in post #174 here.
Having said that … one of the advantages of a component mattress is that you can replace just a single layer instead of replacing the whole mattress if one of the layers softens or breaks down before the others (usually the top layer) or if your needs and preferences change over time.
Phoenix