The lower price of the Mn will drive the price of TRC down at the beginning.
Make your calculations:
1) now there are miners that have already invested into equipment so they don't need to spend a lot of TRCs to cover their expenses and being TRC merge mining they are probably just holding.
2) after the Mn are implemented we all will need to sell a lot of TRCs to cover the expenses for VPS
So halving the price of the Mn will just drive the price even more down.
Maybe the best idea would be to create a high price Mns in the beginning and after some time release the second level masternodes for a lower price (like they do it for XBy).
We made our calculations and lowering the cost of masternodes to 5000 is the way to go.
In response to your assumptions:
1) Miners have been selling TRC on the market from the beginning (some mining pools automatically sell their TRC on the market and give their miners BTC.). Before we decided to add DASH's masternode tech the price was slowly being driven down by miners because we couldn't get enough new investors to buy up the coins sold by miners. There is nothing wrong with them selling their TRC immediately. They want to make money just like everyone else.
2) Even if everyone sell all of their masternode rewards (which isn't going to happen. I know I am going to hold mine and just pay the $20 out of pocket every month) immediately you aren't taking into account new people wanting to get in on TRC masternodes. At the price of 5000 TRC it makes it more affordable and available for new people to join. Right now a masternode is $400, and things haven't even gotten started yet. Imagine if the price of TRC is $.20, or $1.00.
I don't know anything about XBy so I don't want to comment on what they are doing.