As far as I know; we can say that. Because; in certain instances even "الموصوف noun" too may be casted(مخذوف) as well as "adjective الصفة" . There are many instances that "noun" has been casted and meaning still doesn't get ambiguous due to casted noun's pre-cognizability.
As for the given example , منذ is a time-dedicated pronoun; so one can easily recognize that any adjective follows it; is related merely with time. So when we say منذ طويل no one would get it (for example) as منذ رجل طويل other than منذ زمن طويل.
Please let me show you two instances; one for casted adjective and other for casted noun through Holy Qur'an:
1-يأخذ كل سفينة سليمة.Here adjective سليمة is castedمخذوفة.
2-... و عندهم حور قاصرات الطرف... Again here nounحور of adjectiveقاصرات is casted.
you can't casted 'nouns' زمنfrom that sentence , it will have no means . but you can casted 'adjective' طويل منذ is 'ظرف زمان' time-dedicated pronoun so we can say "منذ زمن " and this can handle a "short" or "long" "time" because you didn't specified .
shafaq the example يأخذ كل سفينة سليمة the adjective سليمة is casted but not the noun سفينة "ship" .
"الموصوف noun" here are very different
2.... و عندهم حور قاصرات الطرف..Here there is two 'nouns' حور and الطرف last قاصرات are 'adjective' . حور in this place is 'إسم مضمر' ref ...
I think it's important to recognise in any case that most of the time when people ask whether it's possible to say x or y they want to know about MSA usage, not theoretical possibility based on classical grammar and analogy with the Qur'an. Even if something is theoretically grammatical that doesn't mean it's a good or even understandable translation.
Neither حذف الصفة nor حذف الموصوف are grammatically a “must”; but just “permissible in certain instances” as I mentioned before.
The most important condition for this casting/canceling permition is “the obtained semantic continuity”. The phrase منذ طويل , topic of this thread is a suitable instance for these “certain instances”. If one talks about daily usage; I think منذ طويل is the most common example for الموصوف المحذوف in daily conversations as well as written texts.
As منذ being a time-dedicated pronoun; I wonder; if there is any when heard لم أره منذ طويل will get it as absurd as “Since a long pencil/table/wo/man I didn't meet him.” .....?
It doesn't hurt us if we learn/know/remember that حذف الموصوف و الصفة is a grammatical fact as well as others like .حذف المفعول - حذف المضاف والمضاف إليه .- حذف الشرط وجوابه .- حذف القسم وجوابه and so on .... .
And some versions of that type of canceling/casting are none-rarely encountered in daily conversations and texts as well as rhetoricals.
Just as an example; some times we say just ! يا حبيب instead of يا شخص حبيب!.
And finally; even in English; in certain conditions you may cast/cancel the “noun” -as I did - without disturbing any one.