I don't know - Tolkien himself was certainly concerned about initial sales of the TH
http://www.bbc.co.uk/news/entertainment-arts-19965058
My suspicion is that Tolkien couldn't have written a sequel in the same style - TH itself is 'growing up' by the end & being absorbed into the Legendarium, so it really depends what you consider the 'style' of TH to be. The early drafts of the sequel in HoM-e actually take a backward step & attempt to recreate the early parts of TH rather than the later parts & for that reason don't really fit with the overall mood of the earlier work. LotR starts off very much in tone & mood as TH ended.
My feeling is that if JRRT had produced another book in the style of TH then both books would quite possibly have been long forgotten - yes, the Narnia Chronicles are still around, but many, many childrens' books from that period are not.