I agree with this. If former traditional Labour voters switched to Conservatives then it's not a big stretch for those types of voters to switch to Reform. I would assume most of those types of Labour voters would be OK with that switch as Reform appear from the polls and local elections to appeal to them.
The types of Labour voters that probably won't change are the Guardian type metropolitan voters, most Muslim voters, the voters who are invested in the DEI system and trans etc (which old Labour voters will not be) and those that the unions support like doctors, train drivers, state workers etc. I'd assume they wouldn't want to vote against their own interests so will stick with Labour. Those are specific groups of people with specific interests. I don't think the rest of the country will go for Keir so it will be a numbers game. He's lost pensioners, farmers, anyone interested in preserving our countryside, wealthy people(and their taxes), private school parents, people hoping to pass anything on to their children, the private sector, business owners, kids hoping for a part time job etc.
As to alternatives to Labour and Conservatives, the Conservatives only have Reform whereas Labour voters have a few left/of centre parties to chose from eg Lib Dems, Greens, SNP etc.