Are we mere facilitators of glorifed correspondence courses?

I think you've missed the point.

It is not the responsibility of the instructor to provide ways for the student to learn without coming to class. But there will always be the possibility that one of the students has access to better external resources.

Consider if one of the students is the nephew of the textbook author (or, if you wrote the textbook, the sister of your recent PhD student who extended your work in significant ways after the book was written). It's entirely possible for that student to be mentored outside of class, and become more knowledgeable and practiced than any of the other students, even if the class is taught exceptionally well.

In such a case, you shouldn't consider the student's absence from your lecture a problem that needs to be solved. And a grading system that penalizes this student for poor attendance, rather than assigning a mark based on competence and ability to apply the material, would be a failure.

Some students will use outside resources. It's the instructor's responsibility to devise a grading system that properly evaluates the student's performance, but not to provide the outside resources. Spending your effort on alternative resources dilutes preparation for the class, which harms the students who take the traditional approach.

Focus on handouts, notes, and problem sets that aid in understanding your lectures, and any student who chooses a different approach also has to take responsibility for it.

To answer your question: No, you shouldn't. Yes, it'll be nice if you have time to invest into it.

Ensuring that your lectures provoke in-class discussions with your students and using a Socratic method is a very good thing, and you have my praise. It will, without doubt, encourage students to come to your lectures. However, there will be those who a) orders of magnitude better in learning through reading than through listening, b) who slack and skip lectures, c) who work part-time (depending on the country you're in, this might be acceptable or not). So, do your best to deliver thought-provoking, bright and inspiring lectures, and first cater to those who come to your lectures. There are some ways to help those who don't come; I can think of two.

Personal experience: I've been a TA on a graduate-level physics course. While the professor slides were exactly as you describe (images to seed the conversation) and I liked it, I also felt that the slides will not be enough for me and for the students to retain the content of the lecture. I started writing down the lecture notes (in an electronic format, to be corrected and shared with ease). So, if you are willing to cater to such students but have limited time, the first option is to ask someone to scribe for you.

Another option, which required substantial effort but might be beneficial in the long run, is video recording the lectures. Learn if your university has some kind of video recording initiative; if they do, there will be people who deal with the technical side of recording, and you'll just have to wear a mic. I recall our university did it for a class of 10 people; the recordings were useful when I was preparing for an exam, even for the lectures I attended.

Are we mere facilitators of glorifed correspondence courses?

Unfortunately in the current environment probably yes.

The “chalk and talk” or “sage on a stage” model of instruction is a minimalistic model. That does not mean it is ineffective or old-fashioned. We call it “traditional” because traditionally teachers have to cover a lot of material for a lot of students in a little time, with copious extracurricular responsibilities. Without additional faculty and programme support, what other choice do instructors have?

It is a method we select for efficiency. But if the benefits of being face-to-face are lost when the number of students in the class/lecture exceeds 10–30, then maybe what we have is not even a glorified correspondence course, but a stunted correspondence course. A single lecture is given on a fixed day at a fixed time in a fixed place with fixed seating capacity. In an online environment, any number of students can watch lecture videos in their own time, watch them again as many times as required, pause to check other sources, talk to friends or classmates, take coffee breaks and breathers, and move between effortlessly (depending on the quality of the technology) between different class materials and media.

should I invest more time into developing additional materials that would make it easier for a student to do well in my class, even if that student decides to skip half my lectures?

Ideally, yes. Practically, no.

Three people are responsible for a students’ learning: the student, the teacher, and the teaching institution. Without a institution that supports progressive approaches to teaching and learning, it’s impossible to truly tailor your student-teacher interaction times for individual students’ needs.

If I were a university academic (I am not, I am a secondary mathematics teacher), I imagine it would be wonderful if I could post amazing notes and videos publicly online. If others were doing the same, I wouldn’t even have to produce all the necessary resources myself. Then I could use lecture time as an open communication forum – probably using some kind of technology for students to submit talk items to an agenda. This is probably a bit too progressive (or prematurely progressive) for most tertiary institutions today.

P.S. I very much appreciate your approach to PPT!