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Let's examine the characteristics of the materials used:
Grout is composed of a hard base(silicates/?/etc), a little adhesive, and a lot of water to make it flow. Just like drywall mud, applying a very thick layer guarantees there will be cracking after the water dries.
While the base material does dry rock-hard and sheds most surface water, it does absorb some moisture. Thus, the silicone sealer.
Caulk can be partially latex-based. "Latex" as in latex rubber and/or just thickened paint. Non-sand able but flexes with the surfaces. It is harder to find decent pure latex caulk anymore though because everything has silicone in it. Bathroom caulks at the bottom edge should be pure silicone because of it's flexibility and hydrophobic nature. Problem is, pure silicone doesn't stick well to chalky grout or slick tiles surfaces. Cleaning before applying is essential. Mold and/or rust deposits will still build up on the surface. Harsh cleaning chemicals will compromise the seal.
Using either of these materials as a primary barrier in order to prevent moisture infiltration and build-up(mold) behind enclosed bathroom fixture spaces is limited to their continuity and durability.
Personally, I'd suggest grouting fresh tile in two stages to just below surface level. You could rough up the inside edges with sandpaper as well. Then apply caulk 1/8th inch over the edge of the cleaned tile. You can use painters tape to get a clean line. Work fast and use pressure.
The caulk will eventually fail, but there will be something behind it as a backup.
When re-caulking, cut the line with a rounded but semi-sharp blade and remove the excess off the tile with a soft plastic scraper to avoid
A little more work now will prevent ripping everything out in order to re-joice and plywood the floor later on when everything underneath is rotten.
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