It is interesting that by all evidence, thumri was among the first genres in which the harmonium was embraced: thumri singers were often accompanied and trained by sarangi players, making the sarangi players the core of the genre.
In tracing the relationship between thumri and other musical genres, the harmonium's path into the "big" genre of khyal comes to light. The aforementioned Bhaya Sahib Rao, who was extremely influential in spreading the harmonium, was a son of the maharaja Jayaji Rao Scindia of Gwalior and a well-known singer, and he came from the direct vicinity of the Gwalior gharana, which was the school (even before the Jaipur gharana) with the earliest and closest ties to the theatre.
Amal Das Sharma and John Napier have described the network structure of Bhaya Sahib's gharana spanning learning and teaching relationships, over which he could propagate the harmonium at an amazing rate. Thus it seems that by around 1900, the harmonium established itself as a common instrument associated with the "lighter" genres like thumri, ghazal, and also dadra. The repertoire of thumri singers usually spans several genres, also khyal, so that the inspiration by the development in one genre could influence another. One notable aspect of the harmonium's adoption across musical genres is the fact that this process excluded the transfer of other features even though many women played and propagated the harmonium in thumri, the male dominance in khyal persisted at this time. Manuel notes that many of these khyal singers grew up in the courtesan environment, i.e. they came from families whose male members had the responsibility of training courtesans musically and accompanying them on the sarangi.
Even if the sarangi is still regarded as the instrument "theoretically" most suitable to accompany the human voice, the harmonium today offers a much better alternative for many musicians, solely in terms of tonal colour. The arguments used most often in this context are a certain timbral softness and the possibility to blend the instrumental and the vocal sound through the appropriate selection of reeds. The ideal is to arrive at a blend of voice and accompanying instrument which, in my opinion, is easier to obtain with a free reed instrument than with a bowed instrument, especially in the high register.
The discovery of this affinity between reed and vocal timbre most likely was the prime motivation for high register singers to use and establish the harmonium as an accompanying instrument. The harmonium spread very fast. Whether high or low voice, it is striking that many singers with connections to the courtesan milieu were proteges of the harmonium, and this regardless of genre. In addition to the musical aspect, the sociological perspective again offers an approach: In the 19th century, there was an upward mobility among the sarangi players, i.e. many musicians became more aware of their own musical skills as compared to the vocal soloists skills; the option of becoming a vocal soloist became much more common, while providing sarangi accompaniment moved to the background.