its such a simple and time saving thing.
I'm afraid not simple at all.
To begin with, it can't be a simple brute-force search and replace. At the very minimum there would have to be these exclusions: headers, other links, and plugins--some plugins, like wrap plugin would accept the substitutions, others like tag would not.
Then there is the question of duplicate page names in different namespaces, and overlapping page names, as in the sentence:
There are many complex datatypes.
You might have pages named
complex datatypes [/m]and[m] datatypes. And there is always the problem of scale, some wikis have thousands of pages. You might be able to make use of Dokuwiki's indexer facility, searching for words which are included inside link markup.
Let us know how you make out.