Here in the UK in most cities the amount of empty commercial property is astonishing. You can get space on really good terms eg first 6 months free, next 18 months heavily discounted, and a 5 year lease. A decade ago that would have been nothing free or discounted and a 10 year lease. The reason being that high street shopping is dying. Shopping centres have occupancy rates as low as 80% and are gradually being converted in to complexes of restaurants and bars instead.
Store fronts are an anachronism. Building new commercial property in places where you can't put a service business like a bar is pointless. And very few people want to live above a bar.
Ground level storefront shopping has died a quick death, but I have often seen them converted to offices for small business/freelance/remoting and the outcome is nearly as good. They do not increase walkability much (that should be taken care of by a corner supermarket, a parcel receiving machine and public transport), but it works surprisingly well to keep the "abandoned city" vibe away that haunts purely residential areas during working hours.
In my area, there is an ongoing resurgence of ground-level storefronts with residential over.[1][2]
In both cases, at least one of the rows of homes is actually large condo units on top of ground level retail space. The homes in both developments are selling quickly and in the first, the retail spaces are also condo (owner operated, or whatever you'd call it). It appears to be a mix of light food (coffee shop), retail, a fitness studio, and some small professional offices.
the other significant benefit of mixed zoning is that the city doesn't concentrate office space into pockets and breaks up peak hour travel across multiple directions and locations.
A lovely new apartment complex in the centre of Cheltenham has 10-15 shop fronts available, every single one is empty. Quite astonishing as the apartments seem very popular.
Where I am you can get rent assistance from the council for up to 3 months. We this have lots of businesses that close after 3 months. A moderate newsagent type store on the high street is ~£5k per month (plus business rates, c.120m^2/1400sq.ft) - investors elsewhere don't need occupancy, they've already made back 100% of their purchase price in 10 years. They don't care if the space is occupied. Result, death to the city centre; it's all charity shops and pound shops.
... And like you say more shopping is moving off the high street too. But I'm not seeing the favorable high-street shop rentals you're suggesting.
Store fronts are an anachronism. Building new commercial property in places where you can't put a service business like a bar is pointless. And very few people want to live above a bar.