I agree with a lot of what you say but have doubts about the “powerful calculators”. Certainly I do not think you should assume access to computers is possible. There are many students from less well off backgrounds and we must not assume they all have access to computers as it will exclude them from the syllabus – I used to teach at an inner city comp and know this to be a problem for some. With regard to “powerful calculators” quite what do you mean? I think you should look at the currently available Casio calcs particularly fx-991ES but should not assume students have graphical or programmable calcs as these are generally less good for ordinary mathematical use. Incidentally, I think you should look at the fx-991ES anyway and consider that many students are using this in their pure maths papers!
I’ve taught Decision maths at both the inner city comp and my current very academic grammar school. In the former we introduced D1 as it was more accessible to the less able students than M1, with a resulting increase in results and the students enjoyed it very much (this was the old Edexcel D1). In my current school we have a large number of maths students and give them options for their applied modules (MEI). We use D1 as an alternative to S2 or M2 in the upper 6th and particularly recommend it to those studying computing or business. Our Further maths students all study it. I would be very sad to see it disappear or only available in further maths. For students who find S1 difficult I don’t think M1 is an appropriate follow on option unless they are actually studying physics, in which case they probably should have taken it first.
I introduce D1 by showing an underground map to show them what is meant by a graph and teaching the networks chapter, which is immediately accessible and gives the students a real world application. I find this grabs their attention and interest much more than the algorithms or graph theory, much of which can be taught as you go along. Kruskal’s and Prim’s on a network can be taught completely in one lesson and you have your students in there and fired with enthusiasm. Please don’t drop these!
I hope this is helpful