1) Inconvenience for the riders while the line is closed for "anti-renovation".
As some have pointed out, this can be mitigated....running one track, using more buses, moving one station at a time, etc. Avoiding construction is not really an excuse. The TTC is proposing the same thing for the RT in Scarborough. And look how packed that is now....
2) Engineering risk. The tunnel hasn't been designed for LRT operation. What if it seems as it can be converted, but after the works start, some overlooked incompatibility pops up, and the tunnel cannot serve LRT properly? It will be very hard to go back.
I am an aerospace engineer....and though not a traffic engineer, I would venture to say that any engineer who does not take this kind of conversion into account in the original design is not worth the money at all. Furthermore, any engineer will conduct a detailed design phase before construction starts. They aren't waiting to discover surprises. And if the worst comes to pass, it can still be worked around, subway construction is by no stretch of the imagination, design at the edge of engineering and science.
3) Most importantly, the benefit-to-cost ratio does not look good. I don't know how much the tunnel conversion will cost (probably quite a bit, as apart of fitting pantographs into tunnels, something has to be done about the high-floor platforms at stations). But note that the tunnel conversion, per se, only eliminates one transfer (at Don Mills). If the goal is to create a continuous LRT line across all 416, the cost of tunneling west of Yonge must be added (Sheppard between Yonge and West Don river isn't wide enough for a surface LRT).
We won't know until we study it, will we? Many here and perhaps the TTC are probably afraid of what they'll find...that LRT was probably suitable for all of Sheppard from the start, that conversion can be done for a reasonable price, that recovery rates might improve along the route, etc. Or their study might prove skeptics of the Sheppard subway wrong and end the discussion once and for all. Why won't they at least study the idea? And make the findings public so that we can have a real, informed debate.
If Metrolinx is willing to spend considerably more money on the Finch - Sheppard corridor than the original Transit City plan envisioned, those money will likely generate more benefits for passengers if used for subway extension, rather than subway conversion.
How so? Metrolinx is spending a fixed amount for two LRT lines. If that money went towards subway construction, then it's likely there would be no money for Finch at all, and nothing for Sheppard east of McCowan. LRT is more bang for the buck in this case.
In a dream world, where the province or the feds were willing to spend money on the subway, it'd be no big deal, but if the subway will cost LRT lines throughout the rest of the city, I am not willing to support it. The plan as constructed is probably a fair compromise. I'd support conversion if it was fair priced (250 million or 1-2 km of subway) just to reduce transfer, and increase convenience (lower stop distances). I don't think a few minutes more on an LRV is really all that bad. A few folks might complain but I am skeptical that conversion would result in masses of passengers turning back to the car.