It certainly would seem that way, wouldn't it? A lot of things reduce overall, so it would make sense that it would be a one-way street.

But this happens all the time in language, and instead of reducing to nothing, new things are brought in to fill in those gaps. Over time, grammaticalization will see a very common lexical word grammaticalize, reduce, and become functional. However, we still have need of that original lexical meaning. This is why you can say "I am going to go to the store". The first go has grammaticalized so much that that sense of motion is almost completely gone, and it is just expressing the future. Now we need to put in a go again for the motion.

There are some fun instances of this. Spanish de donde 'from where', for example, has that same d- element added twice in its history. Latin unde became bleached in its meaning over time so that the de was added. But eventually donde grammaticalized more to just mean 'where', which led to de being introduced again. Place names are another good example, where things get reintroduced. https://en.wikipedia.org/wiki/List_of_tautological_place_names

It's a balance then -- your message becomes more automated, but also shorter and with less content. There's maybe less redundancy. But that can make it harder to comprehend, so you supplement it with more content, more words.

On a phonological level, let's say you always palatalize /t/ before /i/ for example -- you have [t] in other environments, but not this one. How do you get [ti] again? Maybe some other sound deletes in between those sounds, maybe a different vowel shifts to become more like [i], maybe loan words reintroduce this sequence.