Hi sfmom,
While many of the two sided mattresses that were made 17 years ago were much more durable than most of the one sided mattresses that are sold in the industry today … I would agree and your “symptoms” also seem to indicate that at 17 years old your mattress is very likely to have reached the end of its useful life.
Whether you are making a “mistake” or not is really a question that only you can answer and it would depend on how the Terra compares to the other mattresses you are considering or that are available for you to consider in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences), on the tradeoff between the time and/or energy you have available to test other similar mattresses in your area compared to how you feel about the price you would be paying, and on all the other parts of your personal value equation that are most important to you.
The Terra certainly uses high quality materials and there are no lower quality materials or weak links in the mattress and I also understand why you like it because it was also my favorite out of the OMI mattresses that I tested as well but as you mentioned it is also in a much higher price range than many other latex mattresses that use similar materials and components so it would be well worthwhile to make some very careful “value” comparisons before buying any mattress in this price range. “Value” is always relative to the other mattresses you are considering or that are available to you and there is more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses.
There is also more about the different ways that one mattress can “match” or “approximate” another one in post #9 here but it’s very unlikely that you will find another mattress that uses the exact same layers, materials, and components so the only way to know whether another mattress would feel similar to you in terms of its comfort/pressure relief, support/alignment, and subjective “feel” would be based on your own personal testing or sleeping experience.
I generally suggest visiting a minimum of two or three stores and choosing a finalist at each of them rather than having all your finalists at a single store and then when you are down to a list of finalists that are between “good and good” and if there are no clear “winners” between them then you are in the fortunate position that any one of them would likely make a suitable choice and then a final choice between them (see post #2 here) would be based on “informed best judgement” and all the objective, subjective, and intangible parts of your personal value equation that are most important to you.
I’m not sure where you are but if you are open to researching other latex mattresses and you have the time and energy and “will” to do so for the sake of finding a lower budget mattress that may be just as suitable for you then if you are in the San Jose area then the better options or possibilities I’m aware of are listed in post #2 here and if you are closer to the San Francisco area then the San Francisco list is in post #2 here.
Phoenix