Canada

There are far too many detached houses with 4+ bedrooms and 2 retired people living in it because the kids moved out

Yeah, they're not the enemy here, and more often than not can't afford to move either. When your retirement plan relies on having a paid-off house to either offset living costs or to have as an asset to sell when it's time to move into assisted living.

Forcing LVT on them means they will need to sell early (and likely quickly) to buy into a condo, which is now going to be FAR more valuable since they will have increased demand (similar to what happened during the pandemic when people were buying houses left and right for the space to quarantine/WFH, houses on my street that sold for $200k a few years before were going for a million+). Subsequently, the property they are selling will be far less valuable due to the tax. So now a retiree is being forced to undersell and overbuy just to avoid a hefty tax bill, pay lawyers, realtors, movers, likely have to sell a lifetime's accumulation of stuff (or abandon it) among all the other costs associated with selling a house and moving. Chances are, they will end up having to pay a mortgage or begin renting possibly decades earlier than when they planned to, eating up their retirement savings.

What do we do with those people? If we left them alone, they would eventually give up the house either when they die, or move into assisted living. It's a very short-lived problem in the long-term, and not nearly as impactful as apartment buildings with 50% occupancy because the landlords want $3k a month or people who own multiple homes as rental properties.

it prevents tax evasion by wealthy people because you cannot hide land, or even work around it by paying yourself $1 or something other financial shell-game bullshit.

How will the value of the land be assessed? Property value is assessed as an estimation of the fair market value if it were to be sold today. Unless it's an empty lot, the fair market value is a homogeneous combination of the land and building(s). To get the value of the property, you would have to subtract the value of the building from that total. This is an estimate that insurers and lenders make all the time and there is little to no real accountability as to how these estimates are made. Say I have a property with a house and the whose thing is assessed at $500k. I take out insurance and they estimate that the replacement value of the house is $600k because that's what they think it would cost to build a similar house there. Does that mean the land it sits on is valued at -$100k?

Maybe instead we try to use recent empty-lot sales to assess land value based on dollars per acre. The only empty lot near me has been vacant for years because nobody has been able to get through the permitting process to build on it and has been sitting with a for-sale sign on it for as long as I can remember. Since nobody wants to buy it, does that make it worthless? If that lot is worthless and it's close to the lot I live on, does that make my land worthless? If I use that to prove that my land has no value, does that mean I pay no tax?

All that still doesn't solve housing for low-income people, though. If I had a plot of empty land big enough to build an apartment/condo building, it would be advantageous for me to build something that I could rent out for as much money as possible, and luxury units are more profitable. If I have a building with empty units, they occupy the same land as the units with tenants, who are covering the tax bill for the land the building sits on. I can then improve the empty units and rent them at a higher rate, because why would I hamstring my profits? LVT encourages me as a landowner to maximize the amount of profit I can extract from that parcel.

I am also not convinced that LVT can be used as a complete taxation system and additionally, I am not convinced that it will address the true shortage of tax revenue, which is that of the extremely wealthy and massive corporations, who extract an amount of wealth far disproportionate to the amount of land they use.