The TGV fares are at least 50% more than the TER fares (around € 110 (TGV) vs € 65 (TER)). However. the TGV takes a bit more than 2 hours, whereas the TER (the train we are talking about here) takes 4.5 hours. This means not daily commutes, but weekly ones. Among the TGVs, half of them are TGV Lyria, which means they run through to Switzerland (mainly Zürich), and it may be sometimes difficult to get seats.

So, there won't be that many people travelling the whole distance. But there are relatively important centers (Troyes, Vesoul, Culmont-Chalindrey) which are not served by the TGV. It is passengers for these destinations which complain (with good reasons).