Languages naturally evolve to be more complex in isolation but tend to simplify when more people learn it as a second language because of the cognitive constraint of adult learners.
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The evolution of a language is greatly impacted by the number of people learning it as a second language. When a language is spoken by a relatively small, isolated community, it tends to accumulate complex features because there's limited external pressure to simplify.
For example, Estonian is a complex language with 16 cases and numerous dialects and subdialects because very few people learn it as a second language.
On the other hand, when more people learn the language as a second language, it usually becomes simpler. That's why Finnish, which is a sister language to Estonian, is simpler because it has more speakers and learners.
Adult learners are the main driving factors behind the simplification of language because they often find it more challenging to master complex grammatical rules and irregularities compared to kids who learn them as their native language. This cognitive constraint, therefore, forces a gradual simplification of the language as it spreads to a broader population.