It may only be enough for conceptual model where you will have to then explain how each concept is a product of the effect other have on it just as in path Analysis and then you may now back it up with theories (Theoretical model)

Busari Yusuf Thank you so much.

It all depends on how ORIGINAL and CONSIDERABLE is your contribution to the field. You don't need to be nominated for a Nobel prize, but it has to be something that fills a gap in the available knowledge, for after all, the goal of a PhD is to train you in order to produce new knowledge.

You need to answer some questions, for example:

1 Why does my research matter?

2 What is the novelty of my work?

3 Why haven't others tried this?

4 How will I assure reproducibility?

5 How do my conclusions will help others in my field?

Ultimately, there is normally a PhD committee that will judge if your proposal is worth a PhD.

I think developing a causal loop diagram (CLD) as a theoretical model may not be enough for the PhD thesis to be accepted unless your thesis answers the questions @Edgar Perez mentioned.