I'm not sure about the Lib Dems TBH but I do still think they should get about 40 MPs (compared with 46 in 1997) even if they don't gain much vote share on 2019. They will probably win some seats they didn't win in 1997 such as Cheadle, Wimbledon, Eastbourne, Wantage, Henley, Woking, Wokingham, Guildford, Esher and Walton, Godalming, St Albans, Berkhamstead/Harpenden plus marginals such as Eastleigh, Winchester, Hazel Grove, Harrogate and Knaresborough, Carshalton and Wallington. Michael Gove and Nadhim Zahawi's seats can't be ruled out for Portillo moments either or even Tunbridge Wells.
This is basically a reverse 2015 scenario, if they gain about 27 seats from the Tories and a couple from the SNP in Scotland.
I think the SNP will be lucky to hold 20 seats now so I will be astonished if the Lib Dems are not the third party even if they underperform.