You must also take into consideration the state of the Japanese RPG. Suikoden V still sold a healthy number of copies, but even at that time, the RPG fanbase was waning. Let's face it, these games are ridiculously expensive and time consuming to develop. It is for this stated reason that Capcom is not working on Breath of Fire VI, and it is also for this reason that a lot of RPGs have gone to portable consoles, where developers not as pressured to make games that look like Final Fantasy XIII.
Overall, while Suikoden IV may have made a minor impact on V's sales, citing that as a core reason is just plain silly as it doesn't take into account all the facts.
I agree, with one caveat: While Konami did not lose money on suikoden V, they did not make too much money either, and this can sometimes be just as bad as losing money. It all depends on the expectations. If Konami tracked sales for SV to be 400,000, then were left with 200,000, this is an embarassment to investors and to the executive staff.
However, the cost of producing a home console RPG is high, but it is not prohibitive. Capcom is a poor comparison, as they have not had the success Konami has had recently. Japanese RPGs were fading during the latter years of the PS2 due to the rise of MMOs like WoW and Everquest. But, recently many MMOs have been struggling, even Final Fantasy XIV. While WoW remains strong, the sales for games like Dragon Age and FF XIII have proven that console RPGs can make money and are far from being extinct.
I suspect that Konami would like to put get their foot in the door on console RPGs, and is simply looking for the best opportunity to do so. With the revival of both Castlevania and Silent Hill for the PS3, I can't imagine them ignoring Suikoden all together. But it is likely that they are wary of making a game that will upset the fan base, and cause sales to be stagnant. Whether SVI comes or not remains to be seen, but at this point it is just as likely to be stalled in the planning stages as it is to not be under development at all.