Recently I have made an interesting observation.
Although it is said that there are more than 80,000 characters in the most complete Chinese character dictionary, the majority are either variants of otherwise common characters or archaic and obsolete characters. According to the 通用规范汉字表 published by China's Ministry of Education, which divides 8105 characters into 3 levels, and according to its official description,
一级字表为常用字集,收字3500个,主要满足基础教育和文化普及的基本用字需要。二级字表收字3000个,使用度仅次于一级字。一、二级字表合计6500字,主要满足出版印刷、辞书编纂和信息处理等方面的一般用字需要。三级字表收字1605个,是姓氏人名、地名、科学技术术语和中小学语文教材文言文用字中未进入一、二级字表的较通用的字,主要满足信息化时代与大众生活密切相关的专门领域的用字需要。
the 6500 characters contained in level 1 and 2 should be more than enough to cover more than 99% of character usage for all media and publications found in modern day China. In fact, some sources would lower the requirement to read 99% to as few as 3000 characters. That said, most native speakers cannot even recognize all 6500.
Now, this should be an accurate representation of the usage of characters in modern Chinese. Looking at Middle Chinese, I find that the characters used in literatures from that era are almost the same as today, with obsolete character being the exception rather than the norm. What I mean is that the essential core set of characters used (most basic 3000 in modern Chinese compared to most basic 3000 in Middle Chinese) are largely the same, albeit some characters having shifted in meaning (走run -> walk、哭cry(shout) -> cry(shed tear),食verb+noun -> noun etc.)
Let's look at some statistics I have gathered (I calculated this myself with a script):
For the entire 唐诗三百首, there are:
Total characters: 23684
Unique characters: 2572
Level One characters: 2065
Level Two characters: 472
Others: 35
["晖","雊","馀","摵","飗","朓","閟","澒","鞿","迳","珓","曈","昽","掎","湓","於","哳","愬","隩","豗","衱","鞚","剺","鄹","鄜","橦","翃","矰","岧","畤","俜","飐","稹","晼","蟢"]
The most common 6500 in modern Chinese covered 98.63% of the 300 poems written in Tang Dynasty, with level one along covering 80.28%. Even among the 35 characters not in 6500 most common, I think 晖 and 曈 are still recognized by most Chinese people today (because of 余晖 and 千门万户曈曈日 which I see every single 新年)
Going back even further to Old Chinese, we have 诗经.
Total characters: 31770
Unique characters: 2747
Level One characters: 1678
Level Two characters: 655
Others: 414
["濩","绤","斁","罍","砠","瘏","痡","樛","藟","蕡","罝","椓","芣","袺","襭","惄","赪","喓","趯","惙","筥","紽","緎","塈","嘒","裯","麕","帨","尨","襛","驺","豝","訧","曀","睆","忮","鷕","卬","笱","洸","褎","泲","窭","泚","篨","浼","鄘","髧","冓","玼","鬒","髢","瑱","揥","晳","瑳","沬","彊","騋","蝃","旟","閟","蝱","芃","僴","咺","槃","薖","褧","蝤","螓","罛","濊","鳣","菼","朅","垝","咥","籊","滺","觿","韘","跂","殳","痗","佸","翿","蓷","暵","仳","歗","吪","罦","罿","漘","啍","襢","裼","狃","掤","鬯","麃","袪","蓵","魗","萚","藘","漙","瀼","儇","緌","蓺","忉","丱","偲","鞹","沵","儦","襋","藚","岵","貆","囷","慆","襮","匊","杕","湑","佽","睘","盬","蔹","驖","俴","楘","辀","靷","馵","觼","厹","韔","縢","涘","駮","檖","枌","鬷","荍","纻","晢","侜","甓","鹝","懰","慅","悁","慱","鬵","祋","咮","鸤","觱","馌","畯","斨","鵙","葽","豜","墐","薁","穋","绹","翛","蜎","蠋","烝","臝","蟏","畽","罭","疐","恌","啴","鵻","骃","韡","矧","酾","藇","羜","糇","饎","禴","骙","旐","痯","罶","鲿","衎","楰","耇","鞗","弨","櫜","炰","菑","涖","軧","玱","焞","麀","麌","晣","厎","絷","蓫","葍","喤","犉","濈","惔","瘥","膴","讻","酲","癙","瘉","惸","蹐","炤","佌","蔌","哿","崒","棸","楀","醘","氵","慭","噂","勚","暬","遹","潝","訿","蜾","蠃","鸴","踧","疢","漼","雊","掎","扡","幠","餤","毚","尰","饛","捄","氿","鞙","璲","桋","疧","湝","鼛","絜","祊","踖","戁","苾","畇","雰","霡","霂","骍","膋","耔","薿","螣","渰","穧","禋","韎","韐","鞸","珌","觩","怲","舝","鷮","殽","籥","嘏","僛","傞","羖","绋","膍","饇","瀌","暱","愒","襜","滮","煁","懆","鹙","蹢","亹","冔","伣","瓞","陾","棫","駾","槱","楛","栵","檿","訽","馘","祃","翯","牣","虡","瞍","淢","嫄","寘","穟","幪","唪","秬","秠","穈","羝","斝","醓","臄","咢","醽","鹥","潨","鞞","泂","餴","墍","昄","翙","菶","憯","惛","怓","瘅","荛","熇","懠","烋","螗","奰","逷","痻","僾","荓","甡","瘗","瘨","崧","粻","鞃","幭","姞","噳","虓","粺","庤","镈","铚","稌","鞉","柷","鲦","雝","鸧","嬛","拚","嗿","畟","挃","紑","鼒","嶞","坰","驈","伾","驒","騢","驔","駜","駽","茷","黮","侐","稙","楅","胾","綅","糦","娀","絿","厖","梴"]
Despite being written over like 3000 years ago(11th to 7th centuries BC), the 2747 unique characters it uses still overlaps 84.92% with the most common 6500 characters today, only 414 not in this list.
This is by no means a comprehensive study and the samples I used are of course VERY little, compared to the VAST sea of literature works produced in Chinese history. However, due to my own limitations, I chose these two as I thought that they're the most representative of the most common characters used in their respective era, and I draw my conclusion that the core set of characters, despite shift in meaning of individual characters, has largely remained unchanged over the centuries. Thus, those tens of thousands of 生僻字 in big dictionaries are either variants of other common characters, or those that were used in that one place for that one era in China, but never gained its popularity and never made it into the core set. Maybe some characters only appears a few times throughout history and that's all, but then the dictionary has to add them.
Am I right on this observation? I am no linguist or 汉学家 so I'd really like some expert opinion on this.