Here’s how I would construct the argument.
Section 1
3 reasons why ARI is a unique immigration category and must be protected from any adverse changes:
1.1. It’s the Portuguese government’s program specifically designed to attract direct foreign investments; it was designed with a particular set of requirements and benefits including the possibility to apply for citizenship under the Nationality Law “in its actual edition”, which were always (and still are today) advertized by the government’s own agencies i.e. formerly SEF and now AIMA, as well as by the Ministry of Foreign Affairs:
Nowhere else and for no other immigration category does AIMA advertize citizenship, except ARI!
1.2. Massive investments of more than 8.65B Euros to-date have been made by ARI applicants. Additional to that figure are approx. 470M Euros in processing fees (which alone are more than x10 times higher than any other category). This was part of the deal designed by the PT govt. in exchange for the package of benefits. The investments were made with the legitimate expectation of a 6-7 year horizon, based on the govt. own advertisement.
No other category is required to make investments!
(this reflects @Luvmhn‘s rationale above)
1.3. Huge processing delays experienced by ARI applicants when dealing with SEF/AIMA. Some applicants as we know are waiting 4+ years for the first card, while at the same time considered legal residents in Portugal (by Despacho n.º 12870-C/2021). Should Article 15(4) be revoked, then all those legitimate years of legal residence will be lost, only due to the faults of SEF/AIMA, but no fault of the ARI applicants.
Again, no other category has suffered such extreme delays!
Section 2
3 points on how / by what means ARI should be protected based on the legal framework in place (thanks to the research by @anon19331392):
2.1. Artigo 18.º da Constituição da República Portuguesa
2.2. Artigo 12º do Anexo do Código Civil , Decreto Lei n.º 47344, de 25 de novembro
2.3. Artigo 167º do Código do Procedimento Administrativo, Decreto Lei n.º 4/2015, de 7 de janeiro
Section 3
3 specific points as to what needs to be protected for ARI:
3.1. No retroactive changes to the Nationality Law in the part of the duration of legal residence required for the naturalization (currently 5 years as per number 1b) of Article 6 of Lei n.º 37/81, de 3 de outubro.)
3.2. No retroactive changes to the Nationality Law in the part of the counting of time period of the legal residence (currently counted from the initial application as per number 4 of Article 15 of Lei n.º 37/81, de 3 de outubro.)
3.3. Continuation of the right to family reunification (I believe for this part an exception for ARI has been already inserted in the proposal of the Foreigners Law (see page 6 of the PDF, clause 2 of Article 98) to be voted on July 16th - so if it comes to pass we should use it as the precedent in our favor and push for the other two exceptions/grandfathering).
And I agree with @PCERoman that when you add your personal story/color to your letter the wording should be toned down, so as to sound assertive and positive but not overly ‘demanding’ or ‘aggressive’ towards the addressees.