Inequality in access always exists, it is caused by the very nature of high-order transit construction. Once you select a route, some riders will be closer to it and some will be further.
It is impossible to eliminate such inequality, but possible to reduce it by spreading out high-order "trunk" lines.
Scarborough subway won't be empty; its load will be similar to that of the western section of BD (west of Jane) or Spadina subway (north of Wilson). The local density in those areas is similar, and the feeder network may even be slightly better in Scarborough (both N-S and E-W routes).
There is no evidence that the desire to raise property values was the main motivation for Scarborough subway. However, it is evident that riders from Scarborough heading downtown prefer to have two transfers (bus - BD - Yonge) rather than three transfers (bus - RT - BD - Yonge).