Hi tsume,
I’m sorry to hear that your mattress didn’t hold up as well as you hoped for.
There are several possibilities why this may have happened including the thickness of your mattress (I’m guessing that you ordered the 5" version?) which may not have been thick enough for you, the weight change you mentioned, the urine or even the regular moisture that we all release during sleep (which can degrade foam more quickly) in combination with the lack of a mattress protector, the effect of the dogs walking or sleeping on the mattress, or even the support surface you are using under the mattress (you can try the mattress on the floor to see if it makes a difference). There are also some suggestions in post #4 here that may also be helpful to extend the life of a mattress that has developed soft spots or is sagging.
While there are many good options that are available to you … their new mattress uses a higher density and more durable comfort layer than their old version (see posts #2 and #6 here) but as you mentioned I would make sure you use a mattress protector on any new mattress. I would also be very cautious about most “traditional” mattresses because so many of them use lower quality and less durable materials in their comfort layers than even the old version of the Tuft & Needle.
Phoenix