I am sure Andii didn't mean Rajang is an ambassador for
hybrids 
... but as a particularly accessible and striking animal he
does raise awareness of oranguatans generally, be they Sumatran or Bornean. At least
some of his visitors
will read the various bits of educational blurb dotted around the enclosure and therefore be educated about matters they otherwise would have remained ignorant of if such a compelling 'attraction' as Rajang didn't exist.
(and yes I am aware Tiga's now there too but maintain Rajang is still the main draw).
I do actually see where Taun etc are coming from and I too visit zoos not just to observe animals but also to support their conservation efforts. However, it isn't as 'simple' as that is it ? With the best will in the world conservation can only be achieved if zoos continue to attract paying visitors and undoubtedly, a large number of visitors come because of a zoo's 'star' animals. So far as Colchester is concerned, Rajang definitely falls in that category, as do Sasha (white tiger), Subu (singular lion) and the 2 amur tigers. Using a similar argument, Subu is 'blocking' a breeding pair from moving into Lion Rock (notwithstanding concerns about its suitability for lions), and Sasha's enclosure could be 'better used' for other breeding cats. In the case of the critically endangered amur tigers, IIRC, although Anoushka had a previous pregnancy which ended in disaster when she ate her cubs, I seem to remember reading that there are no further plans to breed from her. If I'm correct, then perhaps she should 'make way' too ?
Taking this argument to its logical conclusion can you imagine the outcry were Colchester to announce that Rajang (and Anoushka) had been sacrificed for the greater good (as oppose to being euthanised for humane medical reasons) ? Like it or not Rajang is also an ambassador for the zoo itself and arguments about protecting the species wouldn't wash with most visitors. I suspect a great many would vote with their feet and with a loss of income invariably the ability of Colchester to maintain its current care and conservation programmes would be severely affected. Whether you like its various forms of kiddie related entertainment or not, Colchester's general reputation and the way it's branded is that of a family zoo, and getting rid of him would hardly endear the zoo to legions of parents and children, who could hardly be expected to understand the bigger picture. Euthanasia for convenience-sake or even 'conservation-sake' would damage its reputation terribly.
FWIW, I do think euthanasia has its place .... obviously when animals' quality of life is poor, but also when excessive births are unable to be accommodated at their home zoo or elsewhere (obviously, I appreciate this practice doesn't apply to larger more 'noticeable' animals, but I am sure it goes on with smaller, less attractive species when contraceptive efforts have failed). However, to do this to an iconic animal who's lived at this zoo for 30 years and has contributed hugely to its visitor numbers would be very counter productive.