As to why not make the changes mandatory instead of advisory, my guess is that there was an accountant sitting at the table. No disrespect to accountants - they're there to remind you that if you stop making a profit you'll end up on the dole.
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An 'advisory' means that the airlines pay - if they care to. A 'mandatory' means that the manufacturers do.
a guess is one thing, but your supporting statement is not true. Cost between the manufacturer and the customer is a negotiated contractual issue.
Without supporting data it is useless to denigrate a party or person just to support a supposition.
Under continued airworthiness cost is not an issue (alternate means of compliance etc. accomodates this). The facts supporting the product safety issue are not yet in evidence to us although presumably they were to EASA. Just because a system may malfunction does not mean the aircraft is unsafe.