A writing system does not necessarily become less explicit as it ages, and this letter is the standing example of the opposite. Brahmi left the flap to the reader; the northern descendants added a dot beneath the letter and said it outright. The same scripts added an explicit vowel canceller, worked out standard forms for clusters that Brahmi wrote ad hoc, and eventually settled the length of vowels that early inscriptions leave ambiguous on a worn surface. On several measures a modern Indic alphabet tells a reader more than its ancestor did.
That cuts against a common assumption about early writing, which is that scripts start out precise and decay. What actually happens is that a script records what its users need it to record, and the requirements change. Brahmi was developed for a language whose readers knew it, cut by masons on stone for an audience who could supply what was missing. A manuscript tradition copying texts in a language nobody spoke any longer has different needs, and the additional marks appear as those needs appear. The dotted letters of the later scripts are solutions to problems that Brahmi did not have rather than corrections of Brahmi's mistakes.
The dot itself is worth a note. Subscript and superscript dots are the general Indic device for saying that a letter is not quite the letter it looks like, and they are used for at least three unrelated jobs across the family: marking a flap, marking a borrowed Persian or Arabic sound, and in Tamil marking the absence of the inherent vowel. Nothing connects those uses except the convenience of a mark that can be added to any sign without disturbing the order of the alphabet.