Demodectic Rosacea is a rosacea variant, just as valid a variant as Granulomatous Rosacea.
Demodex Folliculorum 
The RRDi is the only non profit organization for rosacea that has officially recognized Demodectic Rosacea as a variant of rosacea. "Recently human primary demodicosis has been recognized as a primary disease sui generis and a clinical classification has been proposed. A secondary form of human demodicosis is mainly associated with systemic or local immunosuppression."  This is referring to a paper published in 2014 "to classify human demodicosis into a primary form and a secondary form."  While acknowledging the work of Dr. Chen and Dr. Plewig, whether you refer to demodicosis or demodectic rosacea we are referring to the same condition. The term 'demodectic rosacea' was coined by Dr. Plewig in an email to the RRDi on March 2, 2007 where Dr. Plewig wrote, "Concerning your questiones, demodicosis can be a disease by itself and thus being independent of rosacea. Or demodex mites heavily colonize pre-existing rosacea and thus lead to demodectic rosacea ( rosaceiform dermatosis). This is a rather complicated issue. Rosacea is usually diagnosed by inspection [of] the eye. Laboratory tests are rarely needed, for instance in gram-negative rosacea, where one needs bacteriology. The same is true for demodectic rosacea, where one has to demonstrate the mites in great numbers." 
Current concepts on rosacea is a video presentation by the Charles Institute of Dermatology, University College Dublin with Frank Powell, MD who interviews Fabienne Fortan, MD, Université libre de Bruxelles, Belgium explaining the latest information on demodectic rosacea:
Demodetic Rosacea has a long history of controversy which continues to this day. For example, note the following quote:
"From these and other statements it is seen that in suggesting the thought that these minute forms of life are etiological factors in even some of the phases of acneform diseases, I shall be but little in accord with the highest authorities. In antagonism to these views, I may say that the results of my observations appear to indicate a close relationship of the parasites with the diseased condition."
Demodex Folliculorum in Diseased Conditions of the Human Face
Proceedings of the American Society of Microscopists, Vol. 8, 1886, page 123, Published by: Wiley-Blackwell
For a comprehensive article on demodectic rosacea and why it is considered a rosacea variant click here.
Dr. Leyda Bowes discusses demodectic rosacea (demodicosis) in this short video:
If your dermatologist dismisses demodectic rosacea you might refer him to this page, the Demodex Mite Videos available for viewing as well as this comprehensive article and comprehensive list of medical papers on this subject.
 Image of Demodex Folliculorum courtesy of National Geographic - by Darlyne A. Murawski
 Iran J Parasitol. 2017 Jan-Mar; 12(1): 12–21.
Human Permanent Ectoparasites; Recent Advances on Biology and Clinical Significance of Demodex Mites: Narrative Review Article
Dorota LITWIN, WenChieh CHEN, Ewa DZIKA, and Joanna KORYCIŃSKA
 Br J Dermatol. 2014 Jun;170(6):1219-25. doi: 10.1111/bjd.12850.
Human demodicosis: revisit and a proposed classification.
Chen W, Plewig G.
 Read end note 7 in the article, Demodectic Rosacea [Variant]