Namaste,
Our sanAtana dharma is blending of culture + philosophies + certain duties that we are expected to do (which most of people do not do it today)
Here the word dharma is understood by many as doing humanitarian work, being good human being, etc, while Sureshwacharya ji in Tai. Up. 1.11 explains dharma as agnihotra - this we do not do it. So the vedic rituals part and vedic study part, oblations to pitr-s etc which is expected is not done by most brAhmaNa-s (brahmins). Similarly, the varNa dharma for xatriya-s (kshatriya-s) is to learn martial arts and administration in addition to veda-s and dharma smriti. Similarly, the ASrama dharma i.e. brahmacharya, etc is also not observed by Hindu-s. In this case how can we ask outsiders to adopt varnASrama (varNa and ASrama) dharma?
To add to it, most westerners would like to continue living according to their culture and progress spiritually. In order to progress spiritually, they adopt the philosophical portions of our SAstra-s i.e. upanishads or yoga sUtra-s, thirumandiram, et al. No doubt they try to change their life style and personal beliefs like believing in reincarnation, etc, but asking them to adopt a varNa dharma would be non-practical, the way they live, as we will need a gurukul and guru-SiShya paramparA.
When we know that 100 % sanAtan dharma cannot be practised, then we should ask them to follow as much as they can follow considering the place, time, situation, etc they live in.
OM